I understand why people think the guards need to "leak" out, but most teams do NOT run fast break offense that way. You can in fact fast break sending 5 to the boards. Watch ANY North Carolina or Kansas game. Not only do they break with 5 on the glass, then run fast breaks after MADE baskets and they are great at it.
Most teams run numbered fast breaks where each player is assigned a lane (there is some interchanging that can, and often does, happen between the 1-2 lanes, and the 3-4 lanes, but the assignments are the same). Ideally, you want to start the break with the PG. They are taught to flare to the sideline nearest the rebounbder (and above the foul line). The rebounder knows the guard is supposed to be there and is taught to pivot that way and throw the outlet as quickly as possible. The other guard must fill the opposite lane. All PGs are taught to take the ball up the side of the court after they cross half court. You are looking to create a 2 0n 1 situation. This is drilled constantly in practice. IF that doesn't open up, the 2 flares to the corner, the 3 (or next best shooter) flares to the other corner, and the 4 (or 5) fills the middle to the opposite low block. The PG then looks to drive, create, or kick to the corner. The last guy down (4 or 5) stays high and is there for a ball reversal.