General

I think he means that if you hold up while performing both Glide Blasts with adequate timing, you leap significantly higher. Sort of as a "Cannon Jump" for Jett. You also get more distance out of your horizontal Double-Glide Blast if perform your first Glide Blast while holding the direction key (as it normally is done), but hold up when performing the second Glide Blast.

Huh? What do you mean a "high jump"? hold alt?