Balk question?

[Hobe]

I have a question on balks that started a debate between umps and coaches in a youth league game. Umps are volunteers without any formal training or experience.

Basic situation:
Have R1 and pitcher is pitching from the stretch, pitcher steps towards 1B and makes throw. The move to first is a slow move, not much intent just moving the runner back.

Ump calls a balk, when asked for clarification Ump states that the pitcher didn't break contact with the rubber as he threw over. Ump states that you MUST break contact with the rubber as part of the throw, his example was a jump pivot or shuffle.

The pitcher's flexibility (youth) allowed him to step toward first and make the throw without much movement of post leg.

My question, is it required that the foot break contact as part of the act of throwing or only that pitcher first stride towards the base. I have read rule 8.05 but have been told that it is the unwritten part that is in question.

If I haven't made this clear I apologize, and I will thank you now for any opinions or official clarifications that are offered.