Instructions -- .22 LR  (NOT .25 ACP)

Here are scans of the Braverman and Stinger Mfg. Co. instruction sheets.  They are a bit different.

Braverman (low quality scan)

Stinger Mfg. Co. (600 DPI scan) 
 

Note that the .22 LR barrel has a tab sticking out that my .25 ACP ADC barrel does not:

 

When the safety is on the breech plate blocks the firing pin, and two grooves (shown) allow the tab on the barrel to pass and the barrel can be screwed on or off:

When the safety is off the breech plate moves (down in this view) uncovering the firing pin and moving the grooves so that the barrel tab cannot pass, and the barrel cannot be screwed on or off:

This safety is not fool-proof -- the barrel can be forced off with a little extra force even with the safety off -- see the scrape marks on the breech plate.

Another idiosyncrasy of this system is once the gun fires the firing pin remains extended until the gun is cocked again.  The extended pin holds the breech plate in the "fire" position, no matter what the control on the body reads.  The breech plate in the "fire" position locks the barrel on.  So you must cock the gun to retract the firing pin to allow the breech plate to shift to "safe" to unlock the barrel to allow it to be unscrewed.

To put this another way.

You just shot it, bang.  You must:

1. Straighten it fully
2. Bend it again to re-cock it
3. Straighten it fully again.