Half the tentacle's max HP, actually (which is 8, so only 4 damage).<br/><br/>And I'm actually rather surprised it took this long to actually kill a tentacle.  It's less effective than killing the thing directly, but if someone had attacked the tentacles holding Tess, she could have gotten free much easier and been able to attack.  (Lyra tried this, but wasn't able to do enough damage.)  The main intent is that you want to keep cutting allies free, because the thing can keep attacking with more and more tentacles otherwise.