Duels?

I've been playing around in RDR for some time now, and my negative thoughts about the game prelaunch has been blown away - the game is just awesome.  
 
But how the hell do you win duels? I've been marking enemies with red and white targets, but Marston doesn't shoot.  
 
Anyone care to tell me really detailed how to win a duel? Or at least making Marston try to win a duel?