Shear a shrub without doing any thinning, and you'll probably allow an increase in size by an inch or more every year. You probably won't even know it as your shears ride above the previous year's level. They simply won't be able to cut into the thicket that results from years of growth all originating at the same point
Thinning involves removing stubs such as those you see here. Cut them back at least a year's growth into the shrub.
Understand that we've spread these clippings to show you that some had leaves, some did not. Truth is, those stubs were not spread out in any way but packed together. We cut out whatever we had to cut to break up the clutter.
So thin for the health of the plant but also to simplify the shearing process and hold the line. Thinned, the shrub will have a greater number of soft, easily-shorn twigs than thick knuckles.
This cut-and-thin process is even quicker when the yew involved is naturally shaped. Here's a yew we've kept this size and shape for 20 years with just 30-minutes of work with hand clippers once per year. More on this in Yews normal cut.