a certain type of ego shooters always features weapons, explosives, vehicles, and somewhat plausible physics. a perfect setting for stunts!
this particulare game doesn't feature Jet Planes so those two guys decided to make a Jet Ski fly; kind of making fun of the game.
i will have to tell you what you see. it's hard to visually decode computer games. whenever i see a new one i can't make sense of what happens on screen either. you have to actually play a game to fully understand what's shown to you, what you're supposed to look at, what you can do, and finally what you should do (and these things change and they depend on what your opponents do).
this is what those two are trying to do: the guy holding the camera has a remote C4 detonator in his right hand. he has to trigger the C4 exactly when the jetski reaches the ramp to give the jetski a boost (we don't see them place the C4. he puts it there before they start shooting).
most of their takes are flawless and the jetski flies impressively far.
you will never see this being used in a regular game though; they certainly can't pull this off with 16 opponents on the island. this is more like two guys messing around with a soccer ball near a goal.
the jetski is hard to see. it has a green rectangle on top. you should watch full screen (danger: music).
you can hear jetski rider say "yeeeep" over the voice com. this helps the other one detonate at the right time. when its perfectly timed the guy riding the jetsky won't even die from the explosion. but twice - the first two times - he dies by crashing into a wall.