This is a very
complex and advanced
vert skateboarding trick, much like its predesscor the
Handplant. If you don't know how to do a
Handplant then don't bother trying this trick until you can.
Once you reach the coping lean forward as you would for a Handplant, but tuck your knees up to your chin. At the same time do the following:
A. As you should already know, don't bail.
B. Place your front hand (left for regular, right for goofy) directly in front of you on the coping.
C. Grab the board in an indy grab as you leave the coping, but this time keep your knees tucked up to your chin.
All you need to do to pull off the Eggplant is keep your knees tucked. You should already know how to dismount from a Handplant, so all you need to do is keep those knees tucked and off you go. The only thing that differs an eggplant from a handplant is the tucking of the knees (you look elongated, like an upside down Eggplant).
Good luck, this is a very difficult trick, though it is still one of the most basic of the plants.