Pitcher throwing to unoccupied base

[parrothead]

So last night was coaching 1B, and was chatting with base ump, said I want to ask you something before I try this pick, and I laid out the exact scenario kind of discussed, runner at 1B, RHP on mound, lifts leg and spins back to 2B, if runner was stealing that it is not a balk because he is making a play on runner and the "unoccupied base" provision does not apply, if runner does not go, its a balk.

He said, he would call balk, that you simply cannot throw to unoccupied base, period. I said "I thought same thing up until 6 mos ago when I saw a coach lose his $100 protest over similar scenario"

His comment to me was something I couldnt quite figure out what he was trying to say, which was the play on the runner provision has something to do with time of pitch.