Action in the Free-Blocking Zone [2-17-2c (NEW), 2-17-4]
Blocking below the waist now requires the block to begin immediately following the snap.

2021 EDITORIAL CHANGES
1-5-1b(3); 2-10-1; 2-10-2; 3-5-1; 9-4-4 PENALTY, NINE-, EIGHT- AND SIX-PLAYER RULES DIFFERENCES – GENERAL; SIX-PLAYER RULES DIFFERENCES – RULES 2 and 7

https://www.nfhs.org/sports-resource-content/football-rules-changes-2021/