Well, my understanding is you have to use the right kind of rope, scrimbonator … Very high quality that will withstand the “crushing” w/o coming apart … That being done, all you have to do is pull it back out when done and move on to the next cylinder … 
I don’t think it’s a wise idea to want to move the valve when there is an obstruction in the cylinder.
Another possibility is to wrap a tie/cord around the valve stem outside the cylinder, in between the coils of the spring and pull that tight.
kind regards
Marek
The whole point is not to move the valve. You feed rope in through the spark plug hole and then carefully turn the crank until the piston pushes it up against the valves to hold them in place while you remove the collets and springs, replace the stem seal, then put the springs and collets back on.
Would be fun to have someone post a video of this maneuver
Thanks for that Russell !
Yes Russell … thanks … I knew I wasn’t hallucinating that method of valve seal replacement … 
And note that the following video illustrates the “compressed air” shortcut method . A few things I can’t figure out though about the first one … (1) he says the compressed air method for some reason is not successful with his old engine, but I’m not clear on WHY … (2) he almost seems to say at the start that the rope method is not going to work on the engine (either), although I’m not clear on why he said that when obviously that method worked … 
(3) wish he gave some up-close video of the rope itself, talked about the type of rope, etc. 
Also, sticking that screwdriver into the sparkplug hole like that makes me nervous … I guess he initially pushes the rope in with it, but it looks like he just leaves it sticking in the hole until the rope is jammed into place ? 
It was the first video I pulled up, there may be more. It just appears to be soft nylon rope, maybe 1/4" and enough to fill the chamber. Neat thing about this is that you don’t have to have the piston at BDC In fact you could put the piston at TDC. With it there, the valve will only drop to the top of the piston if you break the air seal. A magnet should pull it back up unless it is stainless. Not sure what cam lift specs are but can’t be more than 0.50 lift. A good manual may tell the valve to piston clearance when The video shows a tool that will compress the spring. The only problem is that the keepers can be stuck but a slight tap on the side of the valve spring retainer should loosen it.You can put it up halfway and not need so much rope then trun the engine with a socket and braker bar or large ratchet wrench. Yes, the only way to make sure it is filled up is to use something to tamp the rope in. Maybe a plastic dowel would have been better. As with air, as soon as you break that valve seat seal it hits the bottom. There are tools made that will compress the spring but it has to have a mounting point like a cam retainer bolt or bolts. Don’t know about that. All the videos show ways to keep a valve in place. Some good quality some not so good but you will get the point and see the rope or the valve spring compressors.
Hope these might help you.
Balloons wont work unless you had a thick enough bladder but can still get it through the sparkplug hole.
Russ