Once the 4th rax has finished, pull half your SCVs and poke his ramp with your marines.
Pulling means you take them away from what they are currently doing.
So, you pull half of your SCVs away from your resources, but to do what?
This is pointed out in the next sentences:
You can first place a bunker outside his ramp to give you micro space. Once you're up the ramp, build another bunker and start targeting workers. Make sure to keep producing from your barracks and making depots.
Basically, you send some SCVs along behind your marines so that they can build bunkers and repair those bunkers, the other SCVs you have pulled are useful for supply depots and taking damage.
Every time that a SCV dies instead of a Marine when you are pushing him, it's in your advance.
Why does this work? As you are solely producing marines that cost 50 minerals and placing some buildings that cost 100 minerals, you are able to work with half the mineral income and still have constant production running.
When Terran pulls SCVs, it is strongest against zerg; however Mega Rax is also the hardest to hide from zerg.
In contrast with other races, Zerg is the only race that needs to decide whether he needs to spent his larvae on workers or on army units, as he is under pressure he will need to produce more army units at the cost of his workers. So, if you pull SCVs which gets your economy down, your opponent his economy will also be down because of the pressure that you apply on his production...