These implants are harder to get, and they offer situational bonuses that are meant to strengthen specific playstyles.
Battle Hardened

Can be used by MAXes.
Reduces screen shake from nearby explosions and flinch from being hit by bullets by 30% / 33% / 40% / 50%. Rank 5 completely removes flinch and screen shake for 10 seconds after a kill.
Flinch from bullet hits scales with damage they deal. More damaging bullets and headshots cause more flinch.
This implant is useful whenever you get under fire while shooting someone, or something explodes nearby, which happens a lot while playing infantry. It makes it easier to hold the crosshair on the target and offers a competitive edge.
Battle Hardened is popular to the point that it’s considered mandatory by many players, claiming the game is unplayable without it. Especially popular among Heavy Assault mains and other competitive and frontline infantry players, such as CQC BASR players.
The maximum rank is especially powerful and makes an amazing combo with Assimilate implant and HA’s Adrenaline Shield.
Assimilate

Restores 120 / 140 / 165 / 200 shields on a headshot kill. Rank 5: also restores 10% of your maximum ability energy.
Your personal shields will briefly flash when you receive shield restoration.
This implant isn’t very useful without several upgrades, and it’s reserved for the elite infantry players that can consistently get headshot kills.
This implant is not as powerful as it seems. It does nothing for you at maximum shields. While the amount of restored shields is not huge, it can potentially get amplified by Nanoweave and HA’s Resist Shield, and makes a great combo with Adrenaline Shield.
Also, while getting headshots is obviously crucial for competitive infantry playstyle, trying to finish off every enemy with a headshot can be a bad habit. PlanetSide 2 weapons get more inaccurate the longer you shoot, so it’s a common technique to start with headshots, doing quick and strong burst of damage, and then drag the aim to center mass, finishing the enemy with body shots.
Trying to shoot at the head all the time may lead to actually dealing less damage. You will have to make a judgement call each time.
Catlike

Can’t be used by MAXes.
1) Reduces movement speed penalty for crouching by 40% / 45% / 55 %/ 75%.
Normally, crouching reduces movement speed by 50%. Catlike makes you move at 70% / 72.5% / 75% / 87.5% normal walking speed while crouching.
Increased crouching speed allows you to be more mobile during firefights, while having increased accuracy and presenting a smaller target. Especially great when paired with a 75% ADS weapon.
2) Rank 5 also increases jump height by 1m.
This provides a significant increase to player’s mobility and parkour capabilities, even at low FPS. Examples: one, two, three, four.
This implant is especially great for Infiltrators, as crouchwalking while cloaked makes them less noticeable.
It also allows any class to move faster while crouching to avoid being detected by Motion Detection tools.
Increased jumping height has its downsides, as it messes with a lot of accumulated muscle memory and movement habits, so it can be tough to transition to and from Catlike 5.
For example, when jumping into a low window, you have to take a running jump into the window from some distance, as opposed to moving the window and crouch jumping into it.
You can also jump up right in front of it and press forward and crouch while you fall down.
– Atemu
Ocular Shield

MAX units can equip.
Reduces strength and duration of Flash, EMP and Concussion grenades’ effects on you by 15% / 18% / 23% / 30%.
Rank 5 also provides complete immunity to these effects for 5 seconds after using a Med Kit or Restoration Kit (but does not clear effects that were already applied to you).
Sensor Shield

MAX units can’t equip.
Hides the player from motion detection tools further than 30m / 26m / 20m / 10m.
Rank 5: while walking (not sprinting) you cannot be detected at any distance.
Very useful implant for flanking and stealthy playstyles, especially for Light Assaults and Infiltrators.
Critical Chain

Headshot kills grant 20% / 25% / 35% / 50% / faster weapon rechamber speed for 3 seconds. Rank 5: all kills beyond 70 meters also trigger this benefit.
- This implant does not work with shotguns.
- The benefit stacks up to 8 seconds.
- This implant can be equipped by all classes, but only Infiltrator’s Bolt Action Rifles can benefit from it, not counting Archer Engineers.
- This implant effectively turns any sniper rifle into NSX Daimyo.
- It’s possible this implant is currently bugged and doesn’t work with Straight Pull Bolt attachment.
This implant seems to be mostly useful on “long range” bolt action rifles with long rechamber times, when used from a flanking position against unaware group of enemies.
“Close range” bolt action rifles already have very short rechamber time, and often you HAVE to take cover between shots anyway, as excellently demonstrated by the legendary Elusive here.
Heavyweight

MAX units can equip. Reduces the amount of flinch you receive when being struck by a projectile by 50% / 55% / 65% / 80%, but your maximum movement speed is passively reduced by 10% at all times. Rank 5: whenever you take fall damage, you will emit an impulse that knocks back enemy infantry.
Heavyweight provides stronger or equal flinch protection than Battle Hardened at any rank, but it does nothing against screen shake from explosions, which is a big reason to use Battle Hardened for many people. And then there’s movement speed penalty.
You would think that this implant makes a strong option for Light Assaults, but so far there have been no documented cases of effectively using the knockback feature, and in general “taking fall damage” is a very unreliable trigger.
Ransack

Works in vehicles. Killing a vehicle within 25 / 30 / 35 / 50 / meters restores one tick of ammunition for you or your vehicle. Rank 5: also heals your vehicle for 10% of its max health.
This implant triggers for the vehicle you own, and specifically when you kill an enemy vehicle. It doesn’t matter whether you are actually in the vehicle, and you don’t even have to be in the pilot’s seat.
This means, for example, if you are piloting a Harasser while using Ransack, it will not trigger when your gunner is making kills. It will not trigger either if your gunner is using Ransack, because he’s not the owner of the vehicle. So for Harassers – and other vehicles where the owner/pilot doesn’t have a weapon to make kills with – Ransack is effectively useless, unless you are in a squad with your gunner, and let your squadmate pilot the vehicle instead of yourself. Credit to /u/jackch3 for relevant testing.
Bonuses for Ranks 1-4 are absolutely uninteresting, since 1 tick of ammunition is a very small amount for most weapons, and Ammo Printer is a much better choice for that purpose. Overall, it’s hard to recommend using Ransack as it is right now.
Response Jacket

Receiving damage from a standard explosive or light anti-vehicle weapon will trigger a shield that reduces damage from both resistance types by 50% for 6 / 8 / 12 / 20 seconds. Rank 5: also provides same bonuses against C4 and Infantry Rockets.
- You can find lists of weapons using these Damage Resist Types here.
In general, this implant is basically a Flak Armor replacement that triggers only after you actually receive damage for the first time, which makes this implant completely useless in certain situations, and almost as useful as Flak Armor itself in other situations.
Naturally, this implant is mostly useful against weapons that rely on multiple hits, such as ESF Rocket Pods and PPA / Enforcer / Marauder. In fact, this implant is even more useful against them than Flak Armor, because Flak Armor only provides a 20% resistance against them.
Sidewinder

Your strafe speed is increased by 20% / 23% / 28% / 35%, but accelerating and decelerating while strafing takes twice as long. Rank 5: in addition, you no longer appear highlighted on IRNV optics.

This is a great implant for slicing corners. With this implant at Rank 4+:
- Normal strafing is just as fast as normal walking forward.
- Strafing while ADSing with a 0.5x ADS Multiplier weapon is only ~10% slower than with a 0.75x weapon without Sidewinder.
- Strafing while ADSing with a 0.75x weapon is as fast as normal strafing without ADSing and without Sidewinder.
Note that it’s not recommended to ADS until you actually see an enemy, or already know where they are.
The reduction to acceleration speed is noticeable in gameplay, but it’s questionable whether it plays a big difference, since infantry acceleration is fairly fast.
To test: post a screenshot with IRNV optics.