Bingley sent me this:
When a man went down, cries of Habet, Hoc habet! (He's had it!), and shouts of Mitte! (Let him go!) or Iugula! (Kill him!) could be heard. If able, the wounded gladiator would lay down his shield and raise his left hand to plea for mercy, which the crowd signified either by extending their thumbs up or down (pollice verso); turning the thumb upwards and jabbing it toward the heart (pollicem vertere) also was a sign of disapproval, and approval indicated by pressing the thumb and forefinger together (pollicem premere).
From Gladiators
http://ancienthistory.about.com/library/weekly/aa041500b.htm