PC Video Game "Clip tool"?

Alright, so I don't exactly know how to word this, which is probably why I'm getting no google results.

So Im essentially looking for a program that is constantly recording, but not saving that data, until I click some button, and it'll save the last 15 seconds or so of footage.

For example, I managed to get myself in a rare physics-breaking scenario in one of the Open World games I play, and I got shot up into the sky about 100 meters, only to fall back down and die. 

Now, I wasn't recording, obviously, since I wasn't expecting to get sent up 100 meters. but because I wasn't recording, I only managed to pause the game and record the latter half of me falling back down to earth.

I'd rather not have to be constantly filling my storage to the brim with useless footage, or constantly streaming to youtube since that takes up too much of my wifi/bandwidth/whatever. Which is why if it's continually recording, but not saving, it takes up mostly just Ram (I would assume), and only stores the 15 seconds when I tell it to. Sort of like "Recording the past"

So I guess since im supposed to be asking QUESTIONs, my question really is "Doest this software exists, and if so what is its name"

preferably free because I'm too young to work, and I don't feel like bugging my parents. 

If your suggestion is paid, offer it anyway, and I shall sit and await the day I can earn my own wage.

I don't know where else to ask this, so to Yahoo Answers I go