Supplying 5V via USB to a SPIN soldered to a TWIST supplies also TWIST
When powering a SPIN (USB) that is soldered to a TWIST, +5V_DGND from USB supplies also TWIST, in particular PS501
As the power usually comes from a computer, there is no much current hence the output of PS501 is not 12V but somewhat 9V (according to the current capability on 5V).
** Solution:
Add a diode between SPIN and TWIST so SPIN can't power TWIST when USB powered ? (Careful to forward voltage..):