Well, conventional part-writing-- the Palestrina-based rules etc-- has all this momentum back to the tonic all the time. Or, you know, an awful lot of the time. And maybe this is a different experience if it's on piano only, because you can look at that "one" chord anytime and play it. When you sing unaccompanied and the harmonies move in that particular way, you can still figure out where the tonic is, but the usual feeling of that tonic "pulling" you back to it is GONE, because now you're surrounded by notes from keys that aren't closely related to where you were just a few chords back (like key of C, after a few of these moves someone is on a G#). I don't know if I'm saying this very well. I have perfect pitch, and I still felt in singing those pieces, "whoa, where did everything go?" Like I walked around a corner and landed in a whole different neighborhood.