Gomer,
The stick jump test is performed for two reasons:
1) To see if the servos are mis-timed or misadjusted. This means that if the servos are not well synchronized, one might be fighting another, and so when one is turned off, the other will slightly jump, since it has a coorection input already in place.
2) The pedal/collective feedback -
(2A - one servo turned off) if one side of the yaw servo has a fault where it develops little force (blown seals, etc) it will not move easily while the rotor is turning at low rpm, so when you turn the good side off, this bad side will be harder to move, and the pilot's force on the pedals is more easily transmitted to the collective instead of the (higher force) yaw control system. In effect, the yaw force is "reflected back" from the mixer to the collective.
(2B - both servos on) if the yaw cable system has unusually high force (mis-aligned pullys, misrouted cable) then it will have fairly high frictional force, but might not be felt by the leg/foot when making a pedal check at low rpm. An easy way to detect such friction is to see if the pedal motions cause a motion of the collective, because the collective path "reflected" from the mixer is an easier path for the pedal force to go. Any significant motion of the collective while moving the pedals is a sign that the yaw control system needs to be looked at for the source of the extra friction.
Neither of these conditions happens spontaneously, they usually are the result of maintenance of the controls, so MTP's especially should understand the checks to help diagnose faults after maintenance.