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).
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.