Thats a good question.
Well,I think when VDS=0 and VGS>0, then channel in an NMOS transistor is due to the e-s in the substrate.(minority carriers in the substrate.Hence off current of a transistor is pretty low,similar to that of a diode)
However when VDS>0,then we have additional e-s from source contributing to current.Additonally we have a drift mechanism which accelerate holes from source to reach drain.
So , conclusion:
1)VDS<0 and VGS>0, e-s are from p-sub in NMOST
2)VDS>0 and VGS>0. e-s are from both n-source and p-sub in NMOT.
Hope this helps.
PS: I do not have any concrete proof for this but I have discussed with other experienced guys and they feel above explanation is correct.