Who started this or where did it start?
Where I train (Atlanta GA), before rolling, we always "slap hands" and begin. I've trained a little over 7 years, and in my sheltered little world, that's all I saw...
Until earlier this year I went to SoCal for Pans in March. That's when I saw before every match, there was a hand slap FOLLOWED by an immediate fist bump. Peer pressure set in and I quickly added this advanced greeting to my arsenal.
We also have a Yamasaki BB that transferred to our place a little over a year ago, and he does it too...I had previously thought it was just his own cool twist before seeing EVERYONE do it at Pans.
So who started this? Is there some advanced, secret meaning behind it? What makes it better than the single hand slap? 
When my instructor got back from brazil with a few students is when we started to do it.They said they learned it there.
Living in Southern California this is how everyone greets each other; not just in the BJJ world ....
Was fucking weird when I first moved here from the Midwest 4 years ago...
Have no idea how/why it originated, but it's definitely used more then just in jiu jitsu
ohmy23 - Living in Southern California this is how everyone greets each other; not just in the BJJ world ....
Was fucking weird when I first moved here from the Midwest 4 years ago...
Have no idea how/why it originated, but it's definitely used more then just in jiu jitsu
Seriously?
Like if u run into a neighbor at the in and out burger, it's a hand slap fist bump? That's wacky!
Ever get half way into shaking someone's hand, only to see they got the knuckles out ready to fist bump, and u end up palming their fist? I have. Feels so ghey!
I think it's a Brazilian thing. I train in Atlanta as well under a Brazilian and we have always done the hand slap fistie bumparoo.
.
I always wonder how some people know to keep the hand open on the second one and someone bumps it. Mind bottling
I wish there was some kind of sign or warning so I didn't feel so socially inept.
Example: "this is a double slap fist bump mat, please greet opponents accordingly"
You train with Sam "The Crusher" Joseph? 2X Pan Am champ, undefeated MMA fighter and Combat Judo green belt? You are the luckiest man in Atlanta! 
Slap n dap is just how it's done.
i never do this. i simply look at the person i'm rolling with, pretend to do this then shoot a double on the fist bump part...
works every time... the first time.
it's gotten to where people just sprawl in front of me whenever i extend my arm. it's cray.
I always try and grab the slap for a handshake. It's a little game i play. If i miss, I palm the bump, but mainly because I can't make a fist, and it hurts if someone bumps my gimpy half fist.
everyone one i know does this.
in boxing you touch gloves....(fist bump)........ the hand slap was so your opponent couldnt arm drag you from when you shake hands...(gracie self defense) before a challenge fight.
so when the jiujtsu guys started to learn boxing, touch gloves became the knuckle bump.
Bull_in_chinashop - You train with Sam "The Crusher" Joseph? 2X Pan Am champ, undefeated MMA fighter and Combat Judo green belt? You are the luckiest man in Atlanta!
The one and only. Sadly, I have felt the weight of THE CRUSHER! I'd rather have Roy Big Country Nelson on top of me.
Haha The Crusher is awesome!
we slap n dap in hawaii and not even in bjj. at the beach surfing, at the bball court, at the mall, at school we always have done it. just like we invented the shaka which has been a tradition for YEARS. its a hawaiian thing. you guys from socal and brasil come here for a winter to surf our north shore then take our customs back to your lands and claim them as your own. you fuckers steal everything we have ever had. land, waves women and hand signs. fuckers.
Noone from the east coast I train with ever gets ready for the fist bump until they come out to socal for training.
I've never been in a gym out here that doesn't do it.
We do the slap + terrorist fist jab here in Japan too. The instructor went to Brazil to train, so I assume he picked it up from there. I might ask him later.
sufferingsuckatash is correct. It's a surf culture thing that started in Hawaii.
Coach is a Brazilian. He does this. A gringo black belt I know doesn't do it tho. Has any body seen what the new thing is now tho. It's complex. All the black belts instead of fist bumping. One graps the fist after the slap. It gets confusing if ya both wanna grap