Some videos from location tests have surfaced. Check them out here, here, and here. People on the MMC forums, with the assist of BlackKite have gathered a bit more about the Engun system in the game from the updated official site. Now quoting TreIII:
As some of us surmised, the Assist continually just levels up as time goes on, for a max of 100. When you think it’s charged up enough (or you just want some help immediately), press the Engun button, and your dudes will come running out to the background, ready to assist you. Attack power, recovery speed and in some instances, who may be the one to actually help you (if you got two supports) all depend on the level you let the Engun get to.
During normal situations, pressing the Engun button will result in usual support attack.
Pressing it at the same time while being damaged/thrown will result in support counter. Pressing it while doing a special/super will result in support assist to go along with that attack.