A speed sensor is a common part of a modern hydrostatic transmission controlled by an electronic module.
In Sauer Danfoss series 90 pumps and motors, for example, these small hall-sensor-based devices pick up the rotation speed by sensing the alternative magnetic field, created by the magnetized ring that is mounted on the cylinder block. These sensors do not require physical contact with the ring to pick up speed and should be installed very carefully to avoid it, but there is nothing in their construction that prevents a technician from "jamming" them into the speed ring.
If you look at the pictures below, you'll see what happens when the contact occurs - both the magnetic ring and the speed sensor end up in scrap - and those do not come cheap!
You can download the technical information and mounting instructions on Sauer Danfoss speed sensors here. Remember - when in doubt - too much out is always better than too much in.