I know how powerful it is in general. But some semantics have kicked in I am unsure of. The psyker player has successfully dominated an Ork Nob and killed a couple of boyz. However once the enemies were no longer in range, he started using his turns to focus on his psyker (moving then shooting so forth) but keeping the power maintained and not controling the Ork/ignoring the ork under his power. At this point, would controlled characters get their actions back if the possessing psyker does not use his to force actions? Or does he just go limp and fall to the ground until the psyker uses his controlled actions again?

Well remember that every 10 rounds, the psyker has to re-manifest the power to keep it active. So the Ork would remain motionless (he wouldn't go limp unless commanded to by the psyker). Also, the range is cited as 8m, so the psyker should stay within that range or the hold over the Ork's mind will be lost, and remember the normal penalties for psyker powers when you have another power sustained