When a left-handed pitcher attempts a pickoff throw to first base, he must step in the direction of the base before throwing to the base.
This is a good explanation of a legal left-handed pickoff move.
Pitching Rubber Balk
If the stride foot of a left-handed pitcher swings back towards second base and goes past the back edge of the rubber, he must deliver a pitch to the plate. The entire foot must cross over the back edge of the rubber for the balk to be called. It is a difficult angle for the home plate umpire but the base umpire should have a good view.