A jump boost is rewarded when you gain a small amount of boost when landing from a jump of any size, whether its a large jump or a small ledge.
In order for you to trigger the jump boost you will need to release the
button once you are airborne and then press it again just as you land.
Trigger a total of 50 to unlock this trophy, you should not need to purposely work for this as you will trigger many during your playthrough of the game.
You can see how many Jump Boosts you have triggered in 'My Stuff - Statistics & Collection' then press
to go to 'My Badges' and scroll down to Jump Boost.
Here's a video demonstrating a Jump Boost