A plunge line is the line where the bevel meets the ricasso. These are formed when grinding the bevel using a sanding belt, typically a 2" x 72" ceramic belt. Plunge lines can be straight, curved, smooth or sharp. In the video you can see the plunge line being ground down.
Step 1: Mark your grind line
Step 2: make the spot where you want your plunge line to terminate / stop.
Step 3: work at the plunge line and work your way towards the tip.
Step 4: Make sure both sides of the blade are even and consistent so you have a well balanced blade.