Since I teach both yin yoga and restorative yoga I often get a question about how are they different - as both are (very) slow and seem to require a wide arrangement of props. So I'll look at the two practices from the point of view of props:
In yin yoga we are actually working with that edge, we want a gentle stimulation of the deeper tissues of the body. Although many components of the two practices match - being still, staying in poses longer - the first tattva (principle) of the Yin practice is to come into a pose to an appropriate depth (edge) for your body. When coming into a pose, we move to a depth where we first feel resistance in the body, not pushing or striving yet exploring the edge with sensitivity and curiosity. The edge, by the way, can be physical, emotional and/or mental. But as we stay in poses longer, there WILL BE sensations arising. Our practice becomes about opening and being present in the body as our experience unfolds, shifts and changes, developing our capacity to notice and be with what is arising in our body, mind and heart.
In yin yoga the props will also support release and are especially useful/necessary for yogis who are relatively restricted in their movement. Even when I say we are looking for some stimulation for the tissues, we never aim for our maximum capacity or range of motion when we enter a pose. We come into the pose, find our edge, resolve to become still and then aim to relax and soften the muscles, so that the stretch can reach the deeper tissues of the body. More often than not, in order for the muscles to relax, release and soften, we will need to use support - under our knees to relax the thighs, under our seat to be able to tilt pelvis and round the spine safely, under the forehead to support neck.
But there is that ONE difference when using props in restorative and yin yoga - in yin the aim is not to take out sensation completely nor to force the body to pause and stay at that initial edge. You will find, that after a minute or two in the pose, body releases tension and softens, so you might be able to move deeper - not with force, but by responding to the natural opening in the body. That is one of the aims of the practice - to stay mindful of what is arising and then respond accordingly. Let me illustrate that with a butterfly series (hover over the picture with cursor to get to the story of each picture):