The worktable portion is made up of four 2x6 boards with half lap notches cut into them. When the plywood is placed on the worktable, these sacrificial boards are cut through. The cuts are not very deep, so the boards should withstand many cuts. The boards are also inexpensive, so when they become unusable, just make more.
The first step to cutting notches in the sacrificial 2 x 6 boards is to mark out the notch. The notch should be as wide as the 2x6 board it will be joined with and as deep as half the width of the face of the 2x6 board.