This can mean many things, all of which are (as the name implies) destructive to the recipient's anal opening.
Noun: Something which is used to inflict massive anal damage (broomhandle, monster dildo, armadillo, shoe, etc.)
Noun: The actual technique of using a tool (as described above) to pound the victim's anal opening into submission.
"After being tied down to prevent escape, the man was given a thorough backdoor sledgehammer."
"I will use this lampshade as a backdoor sledgehammer if you come near my house again."