The foundational rule, established by Mishnah Makkot 3:5 and developed by Sifra, Kedoshim, Chapter 6:4, is that biblical liability for shaving the beard attaches only to a razor — the instrument that both destroys and shaves — while scissors escape that liability precisely because they cut without the same mode of destruction.
The Shulchan Arukh codifies this distinction in Shulchan Arukh, Yoreh De'ah 181:3, ruling that scissors are permitted even when they shave as close as a razor, though the Rema adds the caution that one should use the upper blade to do the cutting motion rather than the lower, lest it function as a razor; the Bach (Bach, Yoreh De'ah 181) further notes that even the technically permitted close scissor-shave remains disquieting for one who fears Heaven, and that one should leave at least some hair.
רבינו יונה insists that the practice must be to shave entirely with scissors and never to pass a razor at all, warning that those who leave only a thread of hair and shave the rest with a razor are in error; he also records that the scrupulous shave even under the throat with scissors.
With this scaffolding in place, Minchas Yitzchak 4:113 — which should be the reader's primary stop — directly addresses the question of newly invented electric shaving machines that operate differently from the conventional devices, and specifically whether a God-fearing merchant may sell them to observant Jews.