First a comment on coaches. Many are clueless when it comes to what a balk is.
Second
Rule 8.05(a) Comment: If a lefthanded or righthanded pitcher swings his free foot past the back edge of the pitchers rubber, he is required to pitch to the batter except to throw to second base on a pick-off-play. (b) The pitcher, while touching his plate, feints a throw to first base and fails to complete the throw;
You might want to print this out and take it to your coach.
http://mlb.mlb.com/mlb/officia..._rules/pitcher_8.jsp