2.) To ejaculate "knuckle children" (semen) onto a sandwich and then feed it to some unknowing individual.
2.) Jimmy was running his fucking mouth so I gave him a knuckle sandwich.
You see either way its about the same. In example number one you have punched him in his mouth and example two you have made him ingest your hot and salty jizz.
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.