increase lucky fuck odds, apply frenzy, and reduce stamina costs. the first two are kinda good, but the reducing stamina costs kinda sucks as it doesn't reduce heart damage.
Edit: my mistake it does reduce heart damage, it's just impossible to notice because any heart damage after the second wave on endless mode is just THAT insane
Edit2: i messed up again, i was right the first time, endless hearts past the third dragon just DESTROY your stamina, no matter whether they're "tender" or not