There is a button on the transponder in the aircraft labeled "IDENT" that when pilots depress it, instructs the transponder to continually broadcast its squawk code/pressure altitude for 10 to 30 seconds or so (I am not sure of the exact duration). This results in the squawk code/altitude that is displayed on the controller's scope to "bloom," or brighten considerable compared to the other aircraft blips, making it easier for the controller to locate the aircraft on the scope.
Typically the controller will request a "squawk ident" when an aircraft has yet to be identified on radar, as in when a VFR pilot requests VFR flight following or when an IFR aircraft enters/re-enters radar coverage.