Possibly the shaft has jammed. Follow this link for a source of good information: http://www.mapug-astronomy.net/AstroDesigns/MAPUG/ArhvList.htm
From their files:
The locking knob has a gear on the inside end of its shaft. It engages teeth on the outside edge of a large metal disk connected to the mechanism that locks the position of the mirror on the central baffle tube. Rotating the locking knob one direction rotates the large disk to tighten the collet on the central baffle. Reversing direction loosens it.
The teeth do not extend all the way around the large disk. So sometimes the locking knob gear jams to a stop before the lock is fully engaged. Soon after the GPS scopes first started shipping Buck Harley discovered that in such cases you can remove the locking knob with its shaft and gear, then reach in and rotate the large disk with your finger. Replacing the locking knob will now allow sufficient rotation to fully engage the lock. If you happen to move the large disk in the wrong direction, then just repeat the process and move the disk in the other direction.
You don't need to tighten the lock down hard. Just a little snug is enough. You should not attempt to use the coarse focus knob while the mirror is locked. Damage to the focusing mechanism could result.
This design is intended to precisely hold the mirror position. So, yes, it is meant to allow effective use of a separate guide scope by eliminating mirror flop when in the locked position.
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