Miami had just gone up 13-0. KC went three and out, and punted the ball to Wes Welker. It was a bit short, and Welker ran up to catch it. He realized the defenders were bearing down on him, and quickly raised his arm and waved for a fair catch. He caught the ball and was immediately tattooed by Pollard, who apparently never saw the signal, and barely had time to react. Welker was able to hold onto the ball, by the way.
Why was this a smart play? Because Welker knew he was going to get popped either way. But, he took the chance that if he did somehow drop the ball, Miami would retain possession on the fair catch interference.
Miami did go down the field, and Mare missed a Field Goal. But, I think it was a crucial play in the game on Miami's penultimate series of the first half.