On offense you start out controlling the quarterback. Needless to say, you don't need to be a contortionist in order to pull this off. One of the joys of playing offense is using your running back to execute an option play. Get the ball to him in the backfield, swing him towards the outside as though the play is going to be a sweep, then rifle the ball downfield to a wide open wide receiver who'll then high step it into the endzone. Defensively, you start out controlling your safety.
Control is switched to other players by hitting the A button.
The only complaint I have here is that if you want to rush the QB with a lineman, it's next to impossible to switch control quick enough before the ball is snapped. What's great about playing D though is that there's no such thing as pass interference. As you might imagine, this can be turned to your advantage by employing it as a strategy. There's nothing like whacking a wide receiver as he crosses into the middle of the field and taking him out of the play before the ball has ever left the quarterback's hand. It's that much sweeter when you end up burying the QB under an avalanche of humanity for a big sack as a result.