As the description says, press the jump button to jump again in midair. The second boost is substantial and although it does not take the player as high as the Fortified Ankles, it is much more versatile as it can: 1) Be used in combat against melee enemies as a better dodge. Taking it early on can make certain enemies a joke, 2) Be used while falling to reset momentum and take less damage from the fall (there is a threshold to that. After a while, the game will not allow the second jump to trigger), 3) Be used to change direction midair, however slightly, to reach around enemies, 4) (With a bit of practice) be used to quickly take down enemies with the Hidden Dragon perk, which can sometimes be faster than moving behind them and grabbing them. An example of that is when two enemies stand next to each other, it is possible to use the upgraded version of Disable Optics and as it is about to finish uploading to the second one, jump and take down the first, then turn and do the same for the second. Finally, it is way easier to control that the Fortified Ankles which will frequently cause the player to overshoot their destination, resulting in an alert or death. That is not to say Fortified Ankles are not useful, just that they require more practice and are less versatile.

My god this thing is awesome ! it makes you feel like a character from Titanfall.

So if I punch fingers when I first meet him I can't get this?

NOT only at Finger's. You can get this all over

I was under the impression that Fingers gave Legendary version of this. He does not, or at least my game does not. It doesn't matter to punch him or not then since you can get Epic version from anywhere else considering you have the Eddies and Level requirement...