Best guess would be that you are moving the hands to a position that exceeds the limits of the arm joints. With IK the joints do not have any change in value because it a simulation. As soon as you turn off IK then values are placed at each key frame and if a joint is beyond the limits then it will snap back to the limit value, which if the limit is 90 and the IK would place it at 110 the it moves 20 degrees back which causes the jump. Legs have a lot narrower range of movement then Arms so it is harder to move feet to an out of limits location.

Poser 5, 6, 7, 8, Poser Pro 9 (2012), 10 (2014), 11, 12, 13