The valves I have seen with the t-bar you use to adjust pressure, this is how I purge the line.
Say you have been welding and had set at 15CFH flow rate.
Just leave the flow rate adjust knob alone so that you'll be purging out the gas at the 15CFH rate.
When you’re done you want to purge the line. Turn the gas valve off on the tank (not the flow rate adjust).
Turn current to off. That way you don't arc anything since you have to pull the trigger on the gun for the wire feed to go to allow the gas to come out the end of the gun nozzle.
If there is any of the wire spools still left in the machine you could possibly take all tension off so that the wire does not move when you are going the purge the gas in the flow rate valve.
I’m going to see if the tension knob and roller swung out and away as is the way I store it so that the wire is not crushed or the wheel is not deformed and the spring is not being used, can be also allow the motor to turn and not move any wire so there is no waste.
The gas valve solenoid will be activated when the motor turns purging, but no waste of wire on the spool or in the threading all the way out to the gun tip.
If you are only going to waste a few feet with the wire coming out to purge then it is not a big thing anyway. To try to have the wire go back in through the cable would I think be very difficult, not worth messing with.
In any case turn the feed rate to zero or whatever is lowest setting to minimize this loss.
Current off, I would hold the gun an pull the trigger so that the wire goes nowhere near ground clamp or ground potential (just in case the current trys to flow) and watch as the flow rate needle eventually drops to zero.
You can tighten the flow rate to run flow rate faster while pulling the trigger to have the gas purge faster but would not go above the guage flow rate max reading.
Once the flow rate needle is dropped to zero then no gas is flowing release the trigger and turn the flow rate adjust counter clockwise so that there is no resistance and the knob seems like it is going to unscrew and fall off. Stop before that.
Then to be sure it is not so loose it will fall out I tighten till I fell resistance and then back off a full turn or two to be sure that the diaphragm is not being compressed when not in use.
One other thing I heard from an instructor is that when turning the valve on the tank to not have your hand over the top of the handle, but only around the edges to turn. Not that it is likely to happen if the tank threads where the valve is attached or the valve of the tank itself are in good condition, but a valve with 2000 lbs of pressure if malfunction could shoot straight up and out of the tank. Better to have the skin on the hand abrade than no hand at all. Seems like not a bad idea.
PS check stuff posted on the Hobart site for information and the threads there as I have mention as I do believe they have a moderator also that answers some information posted there.