First, get The Union Worker's Guide to Workers Comp to learn how to avoid costing your family hundreds of dollars per week.
Most union contractors don't have light duty jobs. Its heavy duty work like you were doing before the injury or its nothing. But many doctors tell their patients they can go back to light duty, even if there is no light duty work available.
In that case, the insurance company has to pay your full benefits because you cannot return to the job you were doing before you got hurt and the contractor is not offering light duty. Sometimes a contractor will make up a job, like sitting in the trailer or at the job site entrance making sure everyone who comes in has a hard hat and safety goggles - if that's the case, they will pay your hourly rate instead of workers comp benefits.
But if you are making less per week on the light duty job (because you're not doing overtime, for example) you should get workers comp benefits based on the difference.