Electricians – intermediate light switch

I’m swapping some switches at my sons house. The lounge light has 3 switches one of which is an intermediate one. I understand that the L1 & L2 from the other 2 switches go into the intermediate L1 and L2 pairs but how does the intermediate actually switch ie which contacts does it make/break as the switch rocks from one to the other position. It’s all done and work ing but I’ve got my brain in a knot trying to work out the circuit

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