Craig, it is possible to operate the hammer and trigger with the cylinder open if you manually pull and hold the opening latch back to the position it is in when the cylinder is closed. But the latch is spring loaded and you have to hold it back. It is only helpful if you are inspecting a revolver. But Tipoc is correct that the revolver the cylinder must be closed for the mechanism to work and the revolver to fire.

As to the OP, personally I don't mind the extra degree of safety provided by the hammer block.