Expression Map curios

Apologies if I’m missing something really simple…

On the expression maps dialog, can someone please explain why “Add relative channel change action” cannot go negative (to reverse an earlier relative change)?

Context. I’m trying to get GPO5 to trigger a snap pizz as an addition to a KS violin in Aria. It works fine with absolute channels. And it also works with a relative channel, but only if I set the off-event to relative channel 0.

Surely relative channel 0 is the same channel!?

This is illogical. Surely to reverse a relative change of +1 I should apply a relative change of -1?

Thank you.