Rec league using NFHS rules. RH pitcher in set position with R1 on 1B. Pitcher picks NPF straight up and in a continuous motion, without hesitation or feint to any other base, pivots and throws to 1B. Coach hollers for balk, but since NPF didn't break back plane & as described, no balk was called. Having difficulty trying to find the specific language in Rule 6 that says this is illegal. Any help?