This crappy paper shredder uses a mechanical sensor to detect paper inserted, I think how it "works" is the lever arm rotates forward (to the left in the photo) hitting the momentary switch, the motor comes on and the worm gear attached to the driveshaft raises the smooth plate pulling the arm forward again until it hits the switch again, turning the motor off...I'm not sure that's quite right maybe someone could clarify the sequence.
Kinda ingenious design when it works right, except in this model it never really did, the motor always spun too long and now it won't auto-detect entirely. The smooth plate likes to slip out of its position, but I'm unsure of its resting position or how tight the set screw above the spring needs to be to fix it (not sure what else could go wrong...) At least until I finish this shredding job and take it to the Goodwill, that is.
Pics of the arrangement: