This has come up a number of times this year. The rule is this -- if you're engaged with a defender simultaenously or nearly simultaneously when you make a catch, you need to maintain possession through the ground. If you are alone when you make the catch, you need only make a "football move." This is why Troy Brown's strip was indeed fumble -- when the ball was inercepted, the player was untouched and so once he made a "football move" he established possession. If he had been engaged during the catch, he would have had to keep it all the way to the ground. (The rules for "interceptions" and offensive player catches are the same.)
Good explanation.
This play has indeed come up a few times this year -- a notable example being in the Pats/Vikings game on Monday night. In the 3rd quarter, on 3rd and 3, Johnson threw a pass to Wiggins who caught it, was hit in the air (I think by Chad Scott, IIRC), came down, took three steps with the DB on his back, before having the ball stripped and land out of bounds.
The Vikings challenged the ruling, but the play was upheld. The problem was that Joe Theisman spent 15 minutes shrieking about how bad a call it was, how shameful it was that the refs could make that call because Wiggins clearly had two feet down before fumbling.
What he fails to understand is that Wiggins could have dragged the defensive back 50 yards into the endzone, and if he falls to the ground, and fumbles, it will be ruled an incompletion -- when you're engaged with a defended, you need to possess the ball throughout the catch. Two feet down are necessary, but not sufficient, coefficients of a catch.
Jim Nantz, during the PAT after Tomlinson scored, actually explained the rule fairly well, and was even decent enough to give credit to his research team for the information.
God forbid anybody at ESPN make Theisman admit he was wrong.
