What are the implications of "va'Yapeil"?
Rashi: It means that he killed his victim by means of a downward stroke, 1 implying that, if it occurred as a result of an upward stroke, he is not exiled.
If he killed through an upward stroke what is his status? Why is that?
Rav Chisda (Makos 9a): Going upward is closer to Mezid. The Go'el ha'Dam may kill him anywhere.
Rava (Makos 9a, according to Tosfos DH Amar): Going upward is closer to Ones. The Go'el ha'Dam has no Heter to kill him at all.