I found that you can downcast explosion.
What do I mean by downcast?
You press and hold down, and then press jump+[attack] at the same time.
You must have a monster you can hit below you.
However, the monster below you can be out of range of your explosion.
Apparently though, it only works with explosion...
All the other skills force you to do the 'whack the weapon' move when you jump.
I see great use of it in misting.
Looks like third job FP mages directly benefit from this function.
This allows you to increase the downward range of your explosion by at least two times...
(about the equivalent of a Meteo), which is quite useful. xD
Here are some screenshots of me downcasting: