On the far right of the pantograph switches you have a hole when pressing you can couple or uncouple with the key
Same for me, I tried several times with different configurations (cabin on/off, reverser in neutral/forward position, etc) and each time it didn't work.
Ik this is like 4 years after last post was made as I'm new to trains sim world with the tsw4 and just bought the tgv DLC on sale and while looking around trying to have fun with tgv, i hovered pass this hole that had to do with the couplings (haven't messed with it yet) so that is what (this person's quote meant)