This is option is very useful if you want to control your cashflow. It ensures that you won't write a check to a worker until those hours are on an invoice on its way to a client.
If you aren't already in the Payroll Screen, click the Payroll tab in the top toolbar. At the top of the screen under unpaid items heading, click the filter button. On the filter form, check the "only show hours billed to client(s)" option. Click the apply button. Notice you can click the plus signs to expand the workers and you will not see any entries that haven't been billed to a client.
This filter option only blocks hours that haven't been billed to a client, which means that only hours that are in a bill batch are shown. However, this does not guarantee that the hours have been invoiced. Make sure to invoice the hours you bill batch.
If you want to pay all of the workers that have hours which have been billed to a client, you can now click the 
button. Otherwise, first deselect the workers you do not wish to pay. Once the system creates the Payroll Batch, you will notice the new Payroll Batch at the bottom of your screen under the Payroll History heading.
Click the plus sign next to the new Payroll Batch that you just created (it will be at the top of the list). The payroll batch will open up to show you the workers underneath. You can click the plus sign next to any worker to see detailed information on each client for this worker.
The amount you should pay each worker will be in the cost column. You can also see the total in each of the worker details reports, which is created with the payroll batch and accessible from the worker's row. You can also send the worker detail report to your worker by clicking the Email Report to Worker link.