Having cut the shoulders of the tenon with a crosscut saw, Matthew Melton now cuts down the grain to create the "cheeks". This involves using a ripsaw or as he explains a small circular saw will also do the job if you know what you are doing. Following this in the next video he finshes of the oak tenon with a chisel and plane to get a good fit with the mortice

The first stage of cutting a tenon on a large piece of oak to make a mortice and tenon joint. Working deep in the woodlands Matthew Melton demonstrates how to cut the "shoulders" of a tenon using a crosscut saw to cut across the grain precisely. In the next video he uses a circular saw to rip cut down the grain