From the ASIB DVD Commentary

Benedict: The other comic fall was when I'd overstepped my sheet's limit and just fell...

Moffat: He fell like a tree! Because he couldn't move his arms, he couldn't move his legs, he just fell!

Benedict: I was dedicated to the moment.

Lara: Do I dare ask what broke your fall?

Benedict: Uhh.. the carpet.

Moffat: The thing is it was terrible! He falls over, obviously quite painfully, but because it is SO FUNNY everyone just laughs! You hear him go HHHUUNKK onto the floor and then there's Martin going HUMP!