On most of the better (and later) turntables, auto return is done using a non-contact optical system using a LED and photo-transistor or photo-diode to switch a separate arm return motor to return the tonearm. You'll usually see if you remove the turntable bottom cover, they'll have an aluminium plate with a slot (or slots) in it below the tonearm pivot, which passes through between the photo-transistor and LED, and when the tonearm moves, the photo-transistor will see light from the LED when the slot passes by, and will switch the mechanism on. They usually will have a small hole on the top of the turntable somewhere near the tonearm pivot, so that the position the return starts can be adjusted. On fully auto turntables, it will also sometimes adjust the point that the tonearm lowers to start auto play at the beginning of a record. There’s usually a screw underneath the hole that can be turned to do the adjustment.
Sometimes the hole will be underneath the turntable – eg on my Luxman PD-288, it has two screws – one for 45 and one for 33, and each one has to be adjusted through two holes on the bottom of the turntable bottom cover. The Pioneer PL-S40/PL-640 seems to have a screw near the anti-skating dial, although I'm unsure if that's the one to adjust the auto return - it just says in the owner's manual not to touch it, because it's been pre-adjusted at the factory. Try turning it and see what happens! Moving the aluminium plate can also be done to make adjustments to the auto return position.
On cheaper (or older – pre-late-70’s) turntables the auto-return is done mechanically by the turntable motor, using a linkage to push the tonearm directly using a gear from the platter. It’s not such a good method, as the linkages often gum up through grease hardening, and it also puts a slight side force on the tonearm near the end, which can sometimes interfere with the sound quality on extra-long records, if the side force occurs while the record is still playing. It also loads up the normal platter motor or belt, which may not be a good thing, particularly with belt drive turntables. Still, it’s a well-proven method, and cheap to provide, but rather clumsy. They can usually also be adjusted to alter the point at which the auto return starts, but it often can only be done from underneath the turntable, with the cover off. Cleaning and re-greasing the mechanism is occasionally also required.
If the optical system has become faulty, that can sometimes be caused by leaving the bottom cover off the turntable, so that excess light is getting to the photo-transistor to switch it on, even though the end of the record hasn't been reached. Putting the bottom cover back on may cure that.
