Transmission solenoids?

The most common types uses a solenoid in the transmission to route transmission fluid under pressure out through the input shaft into an area at the front of the converter. I don't know of any that use a fully mechanical system unless it was an experiment from the past. When the solenoid opens the fluid pressure moves a piston or pistons depending on the vehicle and that forces the internal clutch to apply. Most use material like is on the internal friction disks on the clutch surface.
There are extreme duty units that add extra clutches to increase the lock-up friction as well as modifications to increase the clutch apply pressures for the same reason.