So, let's say there's a focal mass effect on the right side of the skull, which increases intracranial pressure and squeezes the uncus down onto the brainstem.
Now the uncus isn't only directly compressing the right side of the brainstem, but indirectly, it's also pushing the left side of the brainstem against the free edge of the tentorium forming what's called a Kernohan's notch.