Since noone else replied to our question Ill answer. I figured it out while out shooting my M4.
You load the tube to max capacity. Then pull the bolt/carrier back far enough to add another round on top of the ramp, or shell carrier. After adding that round, hold the same round so as to clear the bolt/carrier as you move the B/C forward just slightly. Then add another round directly into the chamber. If you have the factory Five round tube you now have 7 shots at the ready. Five in the tube, One on the shell carrier/ramp and One in the tube. If you have the Seven round tube then you have a total of Nine shots at the ready.
Make sense?