[papablue]

As long as it's a fluid motion, F1 can lift his free leg up and continue with a step ahead of a throw to 2B. If you judge that R1 has made a valid attempt to steal 2B, then F1's move is legal (attempt to retire a runner). However, if you judge R1's move to be a feint, then F1's throw to 2B is a balk (throwing to an unoccupied base).

John