How does binding "on attack" devotions to relics/medals procs work?

Like for example what happens if I bind “33% chance on attack” devotion proc to “Volley” from Plunderer’s Talisman or to “Blazing Fury” from Pyroclasm Mark?

It will trigger with 33% chance (higher with longer spell timer) every time you do damage with Volley/Blazing Fury.

so if bind it to Blazing Fury from Pyroclasm Mark, which has 12% chance to proc, it means it’s going to proc 33% x 12% ~ 4% of the time??

That’s correct. Binding things to WPS is usually a losing proposition due to their low chance of occurring reducing your control over what procs when. There are exceptions of course, but in this case it would probably be better to bind it to something else.

Thanks for clearing it out, guys.