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The back and forth is used to control a lighting point from two different places (at the top and bottom of a staircase, at the entrance of the room and near the headboard for example).

Diagram of connection of switches connected back and forth

The device used is the same as for the switch. Only the phase wire connection changes: it goes from terminal 1 to terminal L. Terminals 1 and 2 are for shuttle wires (purple). To control light points from more than two different locations, you must use remote switches.
The phase wire arrives at the terminal L of the first switch. Two wires (shuttles) connect the terminals marked 1 and 2 of the two switches. Then there is only to connect the phase start on the terminal L of the second switch.

Video Instruction: How to wire forward-reverse motor control and power circuit.