Haven't been paying too much attention to the spoiler thread this week, so don't know if it's been said, but I'm thinking the attack Luffy was going to use is called "Overdrive".
The term overdrive has a few meanings, which seem to apply to this chapter and Luffy in general. Overdrive on a car is pretty much a term used for the "Top Gear" of the car. The speed is reduced to allow lesser fuel consumption, and provide a better acceleration.
Secondly overdrive can mean to push yourself to excess, or overwork yourself.
In this chapter we see that Luffy has pushed himself to the point where he needs to hold his mouth shut in order to stop G4 from running out.
Lastly the term overdrive can mean to push yourself to the maximum, to achieve excessive activity.
Combine them all into an "Overdrive attack" for Luffy, and basically he has pushed himself to the max, and is going to go one step beyond, and perform his greatest attack. Hence the name will be "Gomu Gomu no Overdrive (Insert name here)"