IIRC, the turning the associated allen wrench rotates a threaded rod extending to the back of the radio case. If you turn the allen wrench clockwise a cog threaded onto the threaded shaft rotates clockwise out of the radio case and into a detent within the rack. Continuing to rotate the allen wrench beyond this point will draw the cog forward until pulled snug against the rack, drawing the radio case deeper into the rack and securing the case in the rack. To release the radio from the rack, turn the allen wrench counter-clockwise until the cog releases the rack and retracks back into the radio case. I don't think it is possible to turn the allen wrench too far clockwise, so I would simply turn clockwise until the radio releases easily out of the rack.
BTW, the above may be the description of some other make/model/brand radio. It has been awhile.