I quote from the FMC user Guide written by Bill Bulfer:
"VNAV PATH uses idle thrust and pitch control to maintain a vertical path similar to a glideslope in 3 dimensions, except there is no ground based beam. The target speed (entered in the FMC) is used for planning purposes only. There is no attempt to to maintain the TGT SPD unless the airplane gets too slow. VNAV PATH is slaved to the path. It is the pilots responsibility to control the speed."
From my own experience if the aircraft accelerates much beyond the target speed whilst trying to maintain the path it will annunciate "DRAG REQUIRED" on the FMC. If it gets much slower than the TGT SPD it adds power to maintain the path (I think!).
Hope this helps.
VBW,
SR