The reason why the sled moves about instead of staying still close to operator’s left front side is the imbalance between the operator and the rig.

After putting on the vest and attaching the arm to the sled and to the vest, you might not be ready to shoot yet. Do one more test to see if the operator and the rig are well balanced. Stand upright, put the sled close to your left front side of your body (for righty operators), let go of your hand, see if the sled could remain vertically still without moving about. If the sled moves about, adjust the balance between the operator and the rig via the two sets of knobs on the arm-to-vest socket and the vest bridge plate.
Side to side adjustment. It is important to remember that the lower knob on the arm to vest socket here mustn’t be wound in adjusting process, that is now screw thread of this knob is seen. It should remain close to the socket block all the time for safety. In case the sled moves further towards the left, turn the upper knob and let the screw thread go out a bit; if the sled moves towards the right side, turn the upper knob and let the screw thread go in a bit.
Fore and aft adjustment. After let go of the hand, if the sled moves quickly forward, adjust the two knobs on the vest bridge plate by turning the lower knob, letting more screw thread out and turning the upper knob and leaving less screw thread out; if the sled moves backward towards operator’s body, do the opposite turning on the knobs. That is, turn the upper knob, letting the screw thread out and turn the lower knob, getting the screw thread in so as to do fine adjustment.
Remember to detach the sled from the arm before adjusting. Turn a little bit at a time and test to see if it’s ok. For the convenience of your future use, mark the thread so you don’t need to do the adjustment again for your next use.