Here' I'll briefly include the uses and types of props in this blog, but I do recommend reading Bernie's article and, when time permits, watching some or all of his video.
The six uses of props in Yin yoga:
1. To increase stress
2. To decrease stress
3. To increase comfort
4. To make certain positions available/accessible
5. To allow us to relax
6. To create length and space
And, the type of props...
4. Belts & straps
In Bernie's article he provides a much more thorough explanation on this topic. Plus he includes a 45 minute video explaining the use of props in almost every Yin pose and discussing the various options available and why these options are of benefit in your yin practice. Here is the link:
Bernie is definitely an expert in his field and I hope you get as much out of reading/watching his article/video as I did.