When you are victorious in battle, you capture the commander and are given 3 options.
Hold, release, execute.
Execute; removes them from the map for 24 hours. But when they do reappear the have double movement + quick march.
Release; instantly teleports them just like what happens right now. And you receive a bonus, I haven't thought of what that bonus might be yet.
Hold; holds them for a maximum of 5 days. During that time you pay upkeep to keep them imprisoned, you also suffer a large movement penalty. You can capture players when capping fiefs but they must be present at the fief to be captured.