tried on arcade stick and....
It seems that it can be done more ways than one, one 6 axis ps3 pad, done the exact way dvdcsn mentioned, though it seems more difficult on ps3 pads... on arcade stick you can do this by charging the lk then release upon release time then as you are releasing you must hit it again at the exact same time as you are releasing..confusing??...
it's basically the same thing but you can release all the way and hit again!!, just look out for some things ....a really good way to do this so you get the best result is to watch jax's arm as it is starting to glow for the pound, just before you see the animation of it starting for the ground(as the arm is glowing but before it starts moving towards the ground)) you release/hit again before he starts to go for the ground pound(repeat process), so basically it's what is called in many other games as a just frame, make sure you are releasing the charge and hitting again before his arm starts towards the ground , this way you can release all the way and then press again, remember to watch for his arm animations, if you do it to late it will not work, if you have an arcade stick try it and work with the timing, i have got 4 ground pounds in a row to work this way, but keep this in mind, using 1 single finger to do this is difficult, i use my thumb and ring finger, 1 to release and the other to hit(repeat)
Edit: this was something that was used back in the day by the "leet" players of mk2 , 1 guy i remember playing against he would play against people using mileena/kitana one handed!!! also is one of the main reasons why jax is 2nd to mileena as far as threat factor goes, seems only right that the 2nd on the tier gets something similar to mileena's sai cheese(sighs).. , also if i remember there was a trick to cut the timing from scorpions spear so you could get it to come out faster 1 after the other, i know for sure you can def get 2 spears in a row for a bit more damage when doing spear uppercut combo, just adding 1 extra spear making it 3 hits ,still catching up with everything in this game, it looks simple enough, but can be very in-depth as well.
dvdcsn, this is yeehaaa on psn, hit me up later for some games man!.