5. When your second is in full possession of the facts, leave the whole matter to his judgment, and avoid any consultation with him unless he seeks it. He has the custody of your honor, and by obeying him you cannot be compromised.

• John Lyde Wilson, The Code of Honor

Wilson was writing in 1830s America, and provides a reflection of the culmination of the role of the Second in the duel, but it certainly to gives a broad view of how the Second functioned in the affair of honor for some time prior as well. In short, the Second was the driving force, and by the 19th century, they were supposed to handle everything, with the potential duelists themselves almost little more than pawns. The duel was being fought over the point of honor, which the duelist was demonstrating they prided over life itself, so placing their honor into the hands of another was no small gesture.

When an affront had been given, the challenger would draft up their challenge with their 'friend' who would then convey the challenge to the other party who would then enlist their 'friend', at which point negotiations fell entirely into their hands, ideally to agree to a compromise that would bring satisfactory end to the disagreement without bloodshed and with honor recognized on both sides or, if unable to do so, agreeing on the terms of the duel itself.

To be sure, it wasn't always quite so neat. Wilson, and other 'experts' believed that the Second must also believe in the cause of their principle, and that, if deciding their man was in the wrong, refuse to continue in the role unless the man make the amende honorable, but this was quite rare To back out of being a second was to imply that your principle was dishonorable, which itself was a challengable offense in theory! And further, while some duels simply were unavoidable due to the depth of insult and rancor of feeling, many seconds simply were incompetent, and a few might have been bloodthirsty themselves, refusing reasonable terms to push the meeting. Lyde, and a number of other theorists of the time, were of the opinion that "nine duels out of ten, if not ninety-nine out of a hundred, originate in the want of experience in the seconds." The Second wasn't there simply for moral support, they were their to ensure that you had an advocate who was both in your corner, but also committed to a peaceful accord, and choosing poorly could be a disaster. One extreme example of this comes from the meeting between William Barrington and a Mr. McKenzie. After exchanging two shots, the men declared that they were satisfied. Barrington's second, a Capt. Gillespie, didn't wish William to make amends and argued with him so hardly that he then drew his own pistol and shot down his own principle! A poor choice indeed...

Of course, a counter example also ought to be made, such as Capt. Lee, who disapproved of his principle's refusal to be satisfied after an exchange of three shots. The other duelist and his second too refused, despite three generally being seen as the furthest a duel ought to go. Mr. Barrow ought to have listened to his friend. Capt. Lee left the field in protest and the duel continued, leaving his man dead on the field on the subsequent fire.

On the ground of the duel itself, the Seconds would handle the loading of the weapons, the marking of the ground, the giving of the signal (this usually decided by lot). Both would themselves be armed and empowered to enforce whatever agreed upon rules were in place for the duel and, if the opposing duelist transgressed them, to shoot that man down in defense of their principle. The conduct of the Seconds was as much on display as the duelists themselves, and just as the duel was a display of manly honor for those standing in the line of fire, ritualistically conforming to the strictures of the code was essential for the Seconds to do the same. After the completion of the engagement, whatever the outcome, it was common for the Seconds to draft a joint statement which recognized the meeting had occurred and that all parties had behaved honorably.

The office of the Second didn't spring up fully formed however, and this role only really fully formed in the 18th century, although it did reflect a similar office that we find all the way back in 15th-century Italy when the duel of honor was first coming into its own. The padrini, of which there might have been one or several for each duelist, was the advocate of fairness for their principle, assist them in preparations, and ensure that their opponent was not cheating such as with a magic charm, or hidden protections, although in that period there was considerably less sense of any negotiations to avoid the duel. By the end of that century though, the Second would be far from an advocate or mere custodian of honor, but rather a full-fledged participant. Becoming the rage in France, and soon imitated elsewhere, the Seconds would simply join in the fight, turning the solitary duel into an affray of two, three, or even four men per side. What was important was that the two sides were equal - there are accounts of a duel being on hold when one man brought an extra friend and a random stranger flagged down to join in and even the two sides. The English aped this practice for a time, but for possibly obvious reasons - it makes the duels much more deadly and in turn brings more wrath of the authorities, but had started to die out by the beginning of the 1700s, with a development -or a return in some ways - of the more subdued role of advocacy.

Sources

See bibliography here, especially Banks.