HSO: What does the Alternation Key do?

I am testing HSO 2 and have loaded a set called Violoncello Solo Combi which has an Alternation key available.

It is G#0 and is red.
While I can play all the expressions provided I cannot get the alternation key to work.

It does not matter whether I press it during or before playing a note. It always sounds exactly the same.
The 20 page manual says very little about it.

(To be clear: I have loaded a set which already has the key set, so I would expect the alternative layers to be present)

What am I missing? Where should I expect to perceive the change?