This is a feature that is available and invaluable to many users, but seldom used. Lets say that you have your clearance Z set to 2" (or more) to clear clamps or other obstacles on the table. This feature uses rapid speed to bring the bit from the "Clearance Z (Z1) down to "Plunge Z" (Z2) instead of Z feedrates. In many cases this is a painfully slow process when using slow Z feeds due to small bits. I usually set my Plunge Z (Z2) to .050" (1.25mm) no matter what my Clearance Z is set at.
Notes: Not all machines can accel/decel at the same rates, so you will have to test this feature to make sure it doesn't cause problems on your machine. The Video Shows the line to add, but there is full information on page 6 of the Post Processor Editing Guide, available from the Help Menu
Here's a 9 minute video showing this feature (and some ATC stuff) in action: https://youtu.be/fHCgR5K0uoM