http://www.cbs.sportsline.com/u/base...ers/rules8.htm
* (c) At any time during the pitcher's preliminary movements and until his natural pitching motion commits him to the pitch, he may throw to any base provided he steps directly toward such base before making the throw. The pitcher shall step "ahead of the throw." A snap throw followed by the step directly toward the base is a balk.
- - - So, that foot has to touch the ground prior to the throw: AND, in the direction of 2nd base for it to be legal.