An extra Adjustable Harness with Carabiner comes in handy when training with the Synapse:
Hands free control of range and speed of movement when using the Synapse in Trainer/Partner mode
Can be used on the wrist for exercises that are limited by grip strength 
Can be used as a ankle/wrist strap for added comfort
Create X-Torso harness ( in conjunction with additional harness)
Connect harnesses to create a long loop to enable additional rotational training