I think it's just operating on each voice separately. And, since the voices are apparently allocated sequentially as you press keys, it glides from the pitch of the key you played 8 (or 16) notes earlier. It's a bit difficult to predict, but i don't really see any other way it could work. Try playing the same key repeatedly, the first 8/16 times the pitch will glide, but after that all voices will be at the same pitch and the 8/16 subsequent notes will glide predicably from that pitch.