I read in a book that casualty rates amongst armored knights were often quite low. In part, this was due to their being better protected, but they were also more likely to be spared because they could be ransomed back to their family for gold, or used in negotiation (IE prisoner exchange).
What I have trouble imagining, however, is how someone would go about restraining a foe during battle. It is my understanding that Kings often traveled with a personal retinue which accompanied them in battle. Did knights have a similar retinue to restrain their prisoners, or was there some kind of etiquette to surrender?
There was no formal etiquette for an individual surrendering on the battlefield. The beaten party was entirely at the mercy of whoever he was surrendering to and mercy was not a guarantee. A French noble, the duke of Alençon, attacked Henry V at the Battle of Agincourt, but failed to kill him. The Duke tried to surrender and was immediately killed by Henry's angry bodyguards. Being taken prisoner could happen in a variety of ways: the Duke seems to have simply recognized that he was outmatched and tried to spare himself an injury, but others, like the Count of Richemont, were only taken captive after being too injured to continue fighting. Even a surrender that was accepted by the enemy did not guarantee the safety of prisoners. There are several cases where medieval commanders deemed it necessary to slaughter the prisoners who had been taken for ransom. This was considered acceptable practice by the conventions of the day.
The exact mechanics of surrender and what happened afterwards seem to be fairly circumstantial. Prisoners were immediately stripped of their weapons and helmet. This helped reduce the risk of prisoners breaking out and rejoining the fight, but it was a very real concern. The purpose of removing a prisoner's helmet was to make them easy to kill if the order was given to slaughter the captives. There also seems to have been efforts made to corral prisoners into separate groups when possible. Several French soldiers were taken captive at Agincourt were kept locked in a house, which was later set on fire. So although ransom was hugely important to late medieval warfare, it also was discarded when practicality demanded.