Jax's MultiSlam move has been confirmed to be in it's simplest form by TRMK correspondent,
SekTrax. Jax slams his opponent a total of five times.
Jax's MultiSlam: LP (Close) - RN+BL+HK - HP+LP+LK - HP+BL+LK - HP+LP+HK+LK
It's easier to press and hold each set of buttons until the next slam is performed, then quickly press and hold the next set of buttons, and so on