Kbd Ctrl may be the wrong tool for what I'm trying to do, but regardless, it's a bit confusing. There is very little info in the manual about what this function does or how to use it (like, two sentences).
So here's what I tried: 3 Parts total... Bass on left of split, Strings and Piano sounds on the right, with the split defined by the Note Limits (key ranges) for each Part. With Kbd Ctrl enabled for all three Parts, everything plays perfectly. So then I wanted to turn off the strings and just hear bass+piano. I hit the Kbd Ctrl icon to disable keyboard control of the Strings part (or so I thought). What happens? Instead of hearing just bass+piano, I hear the opposite of what I want... just strings, no bass or piano. Why? Because turning off the Kbd Ctrl also selects the part which in this case means, not only do you hear it (despite just having tried to turn it off) but you also stop hearing anything else. Note that this is only an issue when turning sounds off. If you're playing just bass and piano, and want to add the string part, you can click the Kbd Ctrl icon to enable the string part and everything will work perfectly (regardless of which Part was selected before you added the string part), you'll hear all three sounds. So this method of mixing and matching sounds (Parts) on the fly from within a single Performance works fine for adding sounds, but as soon as you want to turn a sound off, it seems to kind of fall apart. The solution I found to turn the strings off and just leave the bass+piano playing was to tap the Kbd Ctrl icon to ostensibly disable the strings and then (to actually disable the strings and bring back the other Parts), immediately activate another Part (it doesn't matter which one, as long as it's one with Kbd Ctrl enabled)... and be sure to activate that other Part by tapping somewhere other than on its Kbd Ctrl icon (because if you tap that icon, you will disable its Kbd Ctrl thereby silencing every other sound).
I wish it were possible to click the Kbd Ctrl icon to simply turn keyboard control of a part on or off, without selecting the part (there are other ways to select the part if that's what you want, you could just click elsewhere in the same strip). In this case, it's just doing a bunch of stuff I don't want.. i.e. playing the sound I no longer want to hear, while silencing the sounds I do want to hear. Might there be some way to alter this behavior, or some other way I can select which parts within the Performance I do and do not want playable from the keyboard on the fly? (I know Scenes are another way to go, with their own pros and cons.)