Shimano Revo Shift Gives No Clicks
From the picture, it looks like you have a friction shifter rather than an indexed shifter. Indexed shifters click, and friction shifters move smoothly. For a rear derailleur, you would almost always want indexed shifting because the cogs are so close together. For a crankset with 3 chainrings, friction shifting can be handy for fine-tuning the position of the front derailleur.