What's going on here?

Been having a clear out of old files, found this.

I captured this bit of schematic when I dismantled an old money counting machine, years ago. It's the power supply to an induction motor which rotates the pick-up rotor. The motor is shown by L1, L2 and the circle. Triac D2 looks like a straightforward on/off, but what the heck is going on with Triac D1 + D3/D4?

Any ideas?

Never seen that before, but my best guess is that triac D2 is turned on for normal running, and when triac D1 is turned on there is DC (half wave rectified AC) current flowing in the motor windings and the rotor slows down and stops very quickly.

There is also the possibility that when the DC is flowing, once the rotor is stopped it is locked in position and cannot turn.
(I am not sure about this).

JimB

That would make sense in terms of the rotor picking up notes and chucking them in a hopper, needing to be stopped robustly. I had wondered if it was a way to improve the motor's output in some way, but your idea makes more sense.
I suppose the thing to do is stop being lazy and actually try it out...

Maybe D3/4 form a Dc injection brake.

Yay! It's amazing what you can find out when someone tells you what it's called! https://en.wikipedia.org/wiki/DC_injection_braking