The phase detection circuit is fairly simple. It just makes square waves from the phases and looks at phase A to make sure phase B gets there before Phase C with a simple digital trigger circuit. That way you know if it is safe long before you get the motors moving.
Back in the olden days "computers" (at least the I/O boxes)were really more akin to what you would see in industrial machines with hydraulics moving paper in printers and moving the heads in disk drives. That is where you had the 3 phase motors. Once electronics allowed stepper motors and voice coil head movement we didn't see as many big 3 phase motors but they were still used for disk spindles and such until they got smaller.
The encoders you speak of are used a lot. In the olden days they were magnetic pickups driving a square wave generator, later it was slotted wheels and photo detectors or hall effect devices. In boxes that moved cards or checks they used air sensors to see the card once they got too fast for switches. Optical sensors get dirty too fast.
I really miss the days when things dripped oil on the floor tho. That was when computers were interesting to work on.
The stuff was still pretty fast. A 1403 would print 1100 lines a minute (132 char per line) and that was 1960 technology.
Now it is just "cut open the box and plug in a new one"
