sreten & Alec have got it.
The belt drives the platter which turns the spindle which has a plastic gear that engages the automatic return mechanism when the the return button is pressed or if the arm goes to the end of the LP.
The end of the return cycle is where the big gear has a cutout and will not engage the spindle gear. At that point, the motor stops and the arm lift lever lowers the arm into the armrest.
After taking my PL out of storage, I had to adjust the spring suspension "up" so the platter rides higher. The springs settle with age and may cause the belt to "jump" if too much weight is put on the platter causing the belt to interfere with / chafe the bottom of the platter. Adjusting the platter higher will center the belt on the platter drive area. If adjusted too high, the belt may disengage when switched to 45RPM.
Remove the bottom particle board to get to the springs and plastic housings. Adjusting the suspension is easy, just pull / push, I can't remember which direction, while twisting the plastic housing at each spring. You will feel about 4 or 5 steps of adjustment.
Fred and looking at my PL 115D while posting and getting ready to box it up again because it's been replaced by a Technics linear.
Music is Everything....Except Predictable....WFUV Fan.