Well, I've never actually seen that unit, but doesn't that just push in long enough for you to push it onto the PTO shaft, and then you let go and pull back on it, and it locks onto the shaft? Just like most PTO drive shafts? They often have twisting or pulling mechanisms, but the idea is the same as a button you push allowing you to slide it onto the shaft. That's what it looks like to me, and that it is as it should be.
Is it holding the drive shaft on? I've never used a PTO pump backhoe, so maybe it's not like a regular drive shaft would be.
John