/sit is NOT the same as mashing your X button to both sit and then stand:
Normally if you’re sitting down you would not get Reckoning charges from auto-attacks. However, you could gain charges from random NPCs by constantly sitting and standing up due to server lag: mob would try to attack you as your were sitting, so it’d be a forced crit. However, if you stood up just as you were getting hit, the server would see you as if you were standing and get a Reckoning charge.
^^^that’s from Baconn^^^
the Paladin who wrote the blue stickied Reckoning guide on the original vanilla wow forums - https://web.archive.org/web/20061206141810/http://blue.cardplace.com/cache/wow-paladin/1069149.htm
TLDR - Exploit or not mashing X to gain charges was in vanilla and it was due to spell batching:
originally published on warcraft movies on 2006-02-24 https://www.warcraftmovies.com/movieview.php?id=15930