Originally Posted by kuuan 
the M42 > K adapter locks into the camera's mount, it stays put on the camera, and you screw in and out the various M42 lenses.
If changing to K mount lenses the adapter has to be removed from the camera first.
This is true, though Pentax's explanation of the best way to use the adapter has changed through the years. The manual for my KX (back when the adapters were first made) says to mount the adapter to the lens and then both to the camera. On the other hand, my first adapter (purchased about two years ago) came with instructions that said to mount to the body first and then screw the lens into the adapter.
Either way, unless you modify the adapter for permanent mounting on the lens*, you eventually have to slip it off separate from the lens to remove it from the camera.
Steve
* Modifications for permanent mounting involves:
- Removing the retention spring from the adapter
- Notching/drilling the lens base to engage the K-mount retaining pin
- Some users will add a drop of Loktite to the threads to make the modification truly permanent
Note that such modifications are not recommended for any lens you might want to sell.