When dealing with any fluid system, people refer to things as "upstream" and "downstream". These temrs are in reference to a single point of control. In our case, following the path of the gas, "downstream" is anything after the gas pases through the regulator, and "upstream" is anything before the gas hits the regulator. By regulator, in this case, I mean the infintesimal point where the gas drops in pressure. So, when he says "low side" he's talking about the downstream pressure gauge. This would be the one that reads the lower pressure, look at the gauges, one reads high pressures, one reads low pressures. On the Milwaukee(as well as most other regulators unless they are specially made) the "downstream" or "low" pressure gauge is the gauge on the left(if you are facing the regulator). Conversely, "upstream" or "high" pressure is the one on the right.
And yes, you need to keep positive pressure inside the cylinder or else it can get blow back, which is dust and any other particle that could cause contamination or regulator malufunction later. Anything under 100psig makes it hard for the Milwaukee Reg to maintain good downstream pressure so I like to exchange my cylinders at 100 psig, but the lowest you should ever go is 50 psig.
And finally, regarding "closing" the regulator. What he means is that you need to turn the regulator knob(not the one on the cylinder, the one on the regulator) so that it is all the way counter-clockwise. This drops the downstream, or low side, pressure all the way to 0 psig. You do this as a safeguard to protect your gauges. If you have the knob turned too far in the clockwise direction you can blow out your downstream, or low side, pressure gauge. They don't actually explode or anything, it's just the pressure can be so high that it bends the pressure sensitve spring so that it either A)no longer works at all or B)no longer works accurately.
The gauge is replaceable, for a price.
