At one time this was the standard method for detecting biles whilst legering. After casting out, the line was tightened up and the rod was propped at an angle against a rod rest. Bites were detected by watching for any movement of the rod lip. For the larger species of fish this is still a successful method of legering but the smaller species require the more delicate addition of a line quiver tip. A good quiver tip is also a big improvement when trying for the larger species although you should catch plenty of fish without one.
Bites can vary a great deal from continuous tweaks and rattles to a full-blooded lunging of the rod. When legering in a heavy current the force of the flow will cause the rod lip to bend over. A bite is frequently signalled by the rod tip springing back before lunging down again.
Legering can never be as delicate as float fishing, but used correctly it is a skilful and enjoyable method.