I'm really confused about using sawdust for starting cuttings

Hello,   I was wondering if someone could clear up a point of confusion for me?  

This really isn't a composting question really, but it does involve pure sawdust composting in a raised bed called a "stooling bed".

For those who don't know a stooling bed is a bed that you allow woody plants to become established in while slowly covering the base of the plant with sawdust.  This covering of sawdust mimics soil and causes the plant to send out a root system in the sawdust which is then removed after the root system forms with the new rooted pant then being planted in a proper soil mixture.

I intend to make a apple stooling bed and I've been doing research and it seems that most stooling beds are using what appears to be fresh sawdust at least a foot thick.  

I was wondering how this works?  I've always read that you should never use fresh sawdust or woodchips for any composting.  It's stated that the carbon in the sawdust robs nitrogen from the plant.  Is this just another one of those things that you shouldn't do in theory,  but that everyone does without any real consequence?  I plan on getting the oldest sawdust that I can find and hope that it makes a difference, but maybe there is no problem in this situation?

Thank you for any input.  I greatly appreciate the help.
Have a great day!

I never liked nevers, especially when it comes to growing things.  And books I've seen saying never use sawdust are fine with wood chps...

Sawdust beds for stuff like this have been used for centuries.  So it's worth a go.  But I think it's more like a clamp for storage or how we can grow cuttings in a glass of water.  It's mimicking soil, as you say, not becoming soil.  Compost guidelines apply when we are actively creating soil.

Root growth doesn't need much nitrogen and it takes ages for sawdust to get to the stage where it eats nitrogen.   Depending on tempcand other factors.  I imagine this an over the winter thing as that is what oral historyhas it in our family, so even less need for nitrogen as no leaves.

I have read that this is a technique used by nurseries to make bareroot plants for sale.

I wonder if this really worthwhile for the home gardener?  What does this accomplish?

I see no reason not to use wood chips or sawdust in composting as long as there is a good ratio of green manure and brown manure.

Tim said, It's stated that the carbon in the sawdust robs nitrogen from the plant.

I feel this is correct so it might be best to use the sawdust in composting piles and not on plants unless you are trying to accomplish what the nurseries are trying to accomplish.

In a stooling bed, the shoots that will root are still connected to the mother plant until they've rooted, yes? And once disconnected from the mother plant they get planted in soil rather than sawdust?

I figure that since the mother plant has a mature root system in the soil, which probably extends way beyond the area covered by sawdust, and since the mother plant feeds the shoots with nutrients until they are detached, they probably wouldn't suffer from any kind of nutrient deficiency even if the sawdust did lock up some nitrogen while decomposing. The purpose of the sawdust, if I got this right, is to keep the moisture and prevent light from reaching the part that you want to form roots.

I've observed with cuttings,  if I use good soil, they don't put out enough roots to survive the first crisis.  But poor soil makes more roots.

A lot of propagation uses mediums like sand,sphagnum
moss or perlite that have no nutrition whatsoever.
Mostly they provide water.