I use several different things (and it changes with each territory, like my dq system), but the 2 most common i use are matches ending in snake eyes, and any time someone is 5 or more tokens above their PIN by the end of match. Snake eyes is an auto "1 card injury" with a chance at worse (I use roll 2d6, on doubles the injury adds 1d6 to it's length). Anytime someone goes past 5 tokens over their PIN or Cage rating after a match, I roll a second "PIN" roll. If they are "pinned again" (get their current PIN rating or lower), they are injured for 1d6 cards. there is a lot of extra stuff that can factor in obviously, but those are my basics... keeps me on my toes for creative booking sometimes, lol
I have an expanded feud chart with a ton of options on it and also a bleeding chart that gets used once a wrestler accumulates 3 tokens. I'd be happy to send them to you (or whoever else wants them). If so, PM me your e-mail address.