Why do we slap bump and then roll??
We slap, bump, and roll as a sign of respect to one another and a sign that both persons are ready before starting the roll or match. Its the same concept as a boxer touching glove’s before there match begins. Almost every martial arts has a way to show respect before starting things off.
Is this In the rulebook?
No this is not in the rule book. It is more of an “Unspoken Pact” within the BJJ Community. Sometimes you might see each opponent simply bow to one another before the match beings and there are times I have seen just a simple head nod. But these days you will mostly see each opponent slap bump and start things off that way.
All of these Gestures are a form of mutual respect and an unspoken agreement of a friendly competition, which should indicate “nothing personal.”