Depends on the a/c when it goes into 'on approach' mode. The details are in the B12 section 11. On a 767 it happens (among others) when flap is selected, but not on the 757. Either way it allows VNav Path with the speed window open. It doesn't actually need a notional glidepath either and allows a NPA to be flown in VNav Path, monitored by dme/alt gates.