Clickers are screwed to the center piece in the front area and are used in archery as a "draw length control", ie when the shooter has consistently found his draw length and the arrows have been shortened accordingly, the clicker is used.
The arrow is pulled through under the clicker and when the full draw length is reached, the clicker slips over the arrowhead, which creates a clicking noise, just at that moment it must be released, i.e. release and click are one..., so the shooter has control over the correct release of the tendon.