It is as it sounds: a Man (yes, capitalized for this instance as only a true Man can pull off this maneuver) unzips his pants and takes his dick in both hands and slams it down on the table to prove his point. This is only to be used in circumstances where without slamming the cock on the table, the story, argument, etc. would be laughed off otherwise.
At a sales presentation:
"...and that, ladies and gentlemen, is why you should invest in my company."
"But it makes no sense!"
(audience grumbles and discontent grows)
"Well maybe this will make my point better"
(presenter drops trou and performs the 2 Handed Dick Slam)
Audience applauds and the Man goes on to riches and fame.
A male homosexual variant of the famed Bill Maher Head Slam. A male takes an anal suppository of muscle relaxers to help, loosen up his anal sphincter, his partner dons a nasal respirator and shoves his lubricated head into the anal canal and rectum. Once inside, the man nuzzles and orally stimulates his partner's prostate. Allegedly invented by comedian/drama queen Andy Dick, after hearing of the Bill Maher Head Slam from a male child prostitute.
The Pope doesn't consider the Andy Dick Head Slam, an impure act, because no vagina is involved. But, he added that the Bill Maher Head Slam is totally immoral.