I have just googled this on Benchcrafted. No wonder I was confused, the thing is completely misnamed. According to their site is it an X shaped device compromising two hinged arms, which keeps the bench jaws parallel.
Quick history lesson: St.Peter was crucified upside down, so St Peter's Cross is an inverted cross. You see it on old parish churches of St Peter. St Andrew was crucified on an X shaped cross, which is why the Scottish Saltaire is a white X. Both symbols are well known within the Christian, and especially Catholic, tradition.
I had assumed the St Peter's cross in this device was an inverted cross, somehow bracing the lower part of the leg. Clearly the St Peter's cross in this case was misnamed by someone and in ignorance it stuck.
https://www.classichandtools.com/acatalog/BenchCrafted-Crisscross-Retro-BC-CCR.html#SID=1821