As you can see it is spring loaded on both sides of the release knob.
When you push in the front plunger cap it slides the rod forward locking itself into a detent. This locks cartridge in place.
To unload empty you slide the big knurled knob back, spring pressure pops changer back, cartridge falls out and you are ready to reload.
Works really well and is super fast to change 12ies.
It simply screws in or out to adjust for cartridge size also.....
Others may know something I don't as this is the first one I had my hands on.