Archaeologist Shelby Brown writes about The Roman Arena:
Gladiatorial duels provided the crowd with a direct opportunity to participate in life-and-death decisions. A gladiator who knew he was about to lose his duel would ask the munerarius to pass judgment on him and grant him missio — allow him to be “sent away” (missus) alive. A fighter who was still standing would raise his finger in the air, while one who was down would raise his arm or make some other gesture to signal defeat. A munerarius was expected to turn the matter over to the crowd, which expressed its verdict by yelling or by making the famous gesture generally assumed to be “thumbs down.” The art depicting gladiatorial combat illustrates that referees intervened between the fighters while the munerarius decided what to do. A gladiator who fought well enough to win his duels (or to lose but earn missio) was a good investment, since he knew how to please an audience and survived to fight another day. Under the system of missio, the time and money invested in training gladiators was not wasted after only one combat.The audience demanded courage and respected a good combatant, whatever his social standing. An exhibition of skill or bravery was supposed to be uplifting (Pliny, Panegyric 33), and a gladiator who was not granted missio was expected to kneel with dignity to accept the death-blow. If he pleased the crowd and survived long enough, he could eventually win his freedom. Many gladiators whose chances for a decent life outside the arena were small, or who did not want to abandon excitement and fame, actually reenlisted when they had served their time. A parallel, although a less deadly one, can be drawn with boxing and now “ultimate fighting” to which retired fighters sometimes return. The numerous funerary monuments erected by family members for gladiators who died in the arena indicate that, in the long run, a voluntary gamble with one’s life did not pay. Surviving even ten combats was rare (see Edwards 2007, page 51, listed in the bibliography).