A double onehanded slap to the face with an open hand. You administer it in on motion, first forehand and then a backhand. It has to be two slaps, hence the sandwich. If you start with the backhand and follow up with forehand it's called a "reverse knuckle sandwich" or an "upside down knuckle sandwich". The standard knuckle sandwich is served with the right hand, a left handed knuckle sandwich is sometimes called a "lynx knuckle sandwich".
When hitting the face it is always a good idea to use an open fist. The risk for injury and jail time is less, and it is quite effective. Save the closed fist for the torso.
The bitch needed a good slapping so she served up a couple of hot knuckle sandwiches.
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