Hi DC, I came across this post today while trying to figure out the same thing on my Lynx 2100 LSYB. I was able to look through our factory parameters list (the hard copy booklet that shipped with the machine), and found that the keep relay associated with DSSV just needed to be switched on (it was off, or 0, by default from the factory, not sure why!). The keep relay on our machine was K25.1; that is, keep relay 25, bit 1. Changed this from 0 to 1, and the M516 code works as it should now! After playing around with it a bit, I also noted that it is essential to command a G50 Sxxxx at the tool change or tool path before M516 is commanded, where Sxxxx is your maximum spindle speed for that spindle. For example, our main spindle max RPM is 4500, but I had previously had this particular tool path commanded with a G50 S3500 for a facing path, and didn't want to exceed 3500 RPM. However, this was limiting the M516 function for some reason. I added a G50 S4500 before the tool path where we command M516, and it works as it should. Hope this helps, I feel like I unlocked a secret cheat code on our machine!