and are responsible for that gap if anyone comes through it and they do not chase a man; that's why in the video the guy talks about Nelson "looking for a looper." The RT is responsible for #2 on the LOS (5 technique usually, but can be a 7 or a 9 depending) and the back is helping outside to inside (probably should be vice versa but he doesn't pick up the blitzing safety and either way he's fucked in this scenario).

That's a 4 man slide. In a 3 man slide, the center "turns" the protection and slides in a direction with half of the line (guard and tackle to the side he turns it to) and the other side of the ball is in man protection--guard has #1 man on the line of scrimmage, tackle has #2 and they follow them wherever (preferably working as a tandem to pass guys off if there's loops/stunts/whatever). In a 6 man protection the back will be responsible to either get # 3 LOS or the backer at depth, usually inside to outside first (in other words, if both guys go, he takes the guy with the shortest route to the qb). In a 5 man protection the back is hot if one of those guys blitz.

That's why the qb and center identify the MLB pre-snap. Depending on the protection called they're either sliding to or away from him and it lets the back know where to go.

to my amature eyes. Thanks for the tutorial.

and you can see it easily there, just watch the center.