??? What are you asking about ?
If the Y-stepper-motor, then it mounts to case with 4 screws, thru standoffs.
Those mounting screws adjust belt-tension and can come loose, allowing motor position to rotate.
(The short belt at Y-stepper-motor does not have a tension-spring on it)
One spring on each belt should be enough.
Don't want to Over-Stretch the belts !
Yes, getting them on is Not real easy. ;)