Lavs

I can confirm that turning the engine backwards will prevent the effect of any compression release mechanism.  These mechanisms open a valve slightly during the compression stroke of the engine.  This stroke would otherwise make pulling the engine more difficult.  Once the engine is running, the mechanism disengages allowing full compression.  Even at cranking speeds, however,  the compression does not need to be released during the power stroke of the engine.  In both the power and compression stroke, both valves are normally closed.  By turning the engine backwards, the piston travels up during the power stroke.  Therefore, turning the engine backwards will make the power stroke equivalent to the normal compression stroke, without the interferance of any compression release mechanism. 

This is why Briggs and Straton recommends spinning the crankshaft by the flywheel in the opposite from the normal direction with the normal spark plug installed. A sharp rebound (as it compresses the trapped air since the compression release mechanism is not active in reverse) indicates good compression.  The same method is sure to apply to a PSI test, its just a matter of finding a way to crank the engine backwards.

While this may or may not be a consideration, depending on the engine, I would try turning the engine backwards if low compression was determined cranking the engine normally- just to be sure.