The bottom line is that McNabb was flat on his back, legs sticking up, and Favors threw his full body weight on the back of McNabb's left thigh.
Which is perfectly legal in the NFL:
Digest of Rules
Protection of Passer
1. By interpretation, a pass begins when the passer -- with possession of ball -- starts to bring his hand forward. If ball strikes ground after this action has begun, play is ruled an incomplete pass. If passer loses control of ball prior to his bringing his hand forward, play is ruled a fumble.
2. When a passer is holding the ball to pass it forward, any intentional movement forward of his arm starts a forward pass. If a defensive player contacts the passer or the ball after forward movement begins, and the ball leaves the passer’s hand, a forward pass is ruled, regardless of where the ball strikes the ground or a player.
3. No defensive player may run into a passer of a legal forward pass after the ball has left his hand (15 yards). The Referee must determine whether opponent had a reasonable chance to stop his momentum during an attempt to block the pass or tackle the passer while he still had the ball.
4. No defensive player who has an unrestricted path to the quarterback may hit him flagrantly in the area of the knee(s) or below when approaching in any direction.
5. Officials are to blow the play dead as soon as the quarterback is clearly in the grasp and control of any tackler, and his safety is in jeopardy.
http://www.nfl.com/fans/rules/protectionofpasser