Just for fun, I looked up how the old-school guys did it during the 3-lift era, and it appears they mostly jerked with thumbs but pressed thumbless. Bud Charniga concurs (for what that's worth....he's not necessarily right, given his tendency towards extremes) and cites a Soviet paper, (Rodionov et al.) concluding that thumbless was the way to go (but like Bud, Soviet sports scientists are not necessarily right either).