If one model slays the other, then any excess wounds they inflicted above and beyond those needed to slay the opponent, up to a maximum of +3, are counted towards their side's total number of wounds for close combat resolution.

Note that this is an exception to the rule stating that a model can only suffer as many wounds as it has on its profile. This time you need to add up all of the wounds inflicted on the victim, even those from a weapon causing multiple wounds, or by repeated Killing Blows (each successful Killing Blow scores the same amount of wounds the slain character has on its profile), etc. In addition, should one of the combatant be slain before either the opposing character or their mount has had the chance to fight, they may still make their full amount of attacks for the purposes of Overkill.