How it works here, tested by grievance and varied by BA...
Drivers who voluntarily work preload can still file 9.5 grievances but preload hours do not count towards 9.5 hours.
The principle is you can not simultaneously volunteer for more hours the file a grievance for excessive hours,
It works out well for drivers who want to be off at a decent hour for family stuff but also want a larger paycheck. But since preload hours happen before driving hours, senior drivers get the chance at the extra work regardless of being on the 9.5 list.
You can also volunteer to work, ,say, a 6th shift Saturday, even though you don't want the OT the oth er days of the week.