The proc rate is calculated on a per-swing basis - you can check the implementation if you like. Note - this is Secret's version.
https://github.com/SecretsOTheP/EQMa...tack.cpp#L3721
https://github.com/SecretsOTheP/EQMa...pp#L3809-L3814
After a single swing, either it procs or it does not proc. There is a probability that it procs, and it is calculable and knowable. That number is 10.4% with this weapon.