I have analyzed the plates and a lot of other evidence, and my conclusion is as follows:
My forthright opinion is that the Sassanid release is NOT a fingers based release. This was an idea put forth by ADH Bivar (RIP) in the 1970's, and it needs to follow him into the grave.
If we take the plates at face value, and assume that they are actually representing a genuine technique, then the Sassanid release is a THUMB release. Index finger on top of arrow, thumb wrapped around string, middle finger locked over thumb. Very similar to some varieties of Chinese release.
https://www.youtube.com/watch?v=szEZocZySmU