This question was related to the "adductor spasticity" video:
Casey Clark from Glens Falls Hospital: is the theraband held down by being tucked between the groin strap?
No, it is simply wrapped around the actuator and behind the patient’s buttocks, in one long loop.
If you find that theraband rides up, you can use one or 2 bungee cords: if using one, simply hook it into the plastic buckle (where the groinpiece buckles into the harness) on one side, wrap it around the actuator, and hook it into the groinpiece buckle on the opposite side. If you are using 2, hook each one into the same locations (one on each side, where the groinpiece buckles into the harness), and anchor each one to the handlebar or handlebar mount (set handlebar height so bungee cords are essentially horizontal.