When the enemy army is banging at your door and they’re going to flood you with everything they got; that’s when you rally your squadmates and BRACE for impact. Collapsing on point means getting your entire squad to turtle around the point and fight. You will need every single man you can get because you want to make it so that you have more bodies then the enemy does before the timer goes down. Generally, this should be done within the last 20-40seconds in a point hold and you feel like all hope for taking the base is dwindling unless you get your entire squad together with you. Collapsing on the point may give that last boost of needed cohesion and is one of the ultimate forms of teamwork in point-holding. The squad that does it right and effectively will always have a better chance against a larger uncoordinated force.
An example tutorial video from me about collapsing on point and other examples for section G.
Here is another interesting video where you just have a hunch that collapsing on point is needed. After you gain some massive experience and realize something is wrong on the map. I did it in this instance because I knew that the NC had a connection to this base as well and could drop on it to steal at any time that I told my squad members to collapse early.