First read this:
http://users.telenet.be/elvo/3/2.html
Then you know how a normal piston works, and that it's all about 'pack'.
Ghea pistons have one tapered side. Oil flowing around this tapered side isn't as turbulent, so it will result in less pack. Oil flowing around the non tapered side will be more turbulent and this will result in more pack.
So, if you put the tapered side DOWN, the damper will have more pack when you compress it quickly, than when you decompress it quickly.
On a smooth track with big jumps, you will want to choose pistons with small holes, with lighter oil, to have enough pack when landing from the big jumps so your chassis doesn't slap the ground.
On a bumpy track without big jumps, you will want big holes with thicker oil, so you have less pack, and it will soak the bumps a lot better.
But what if you have a bumpy track, AND big jumps? Then you want to have enough pack while landing from the jumps, and less pack on the bumps.
One way to achieve this to a certain degree is by tapering the bottom of the holes, or the side of the piston (=GHEA). The shock will lock up enough when landing, but it will be able to extend with less lock to go over the bumps (so not quicker, but with less lock, so further). Of course it will not be as good over the bumps as with bigger holes + thicker oil, but setup is a compromise! With big holes your car won't be as good on the jumps...
For example with losi pistons:
You want to use red pistons for the bumps, but black pistons for landing jumps. Then choose GHEA black pistons, and you will have the pack of a losi black when landing, and a 'similar' feeling of losi red when riding the bumps.
Imo, the ghea pistons excell on high grip, bumpy astro tracks with jumps. Like almost every track you have in the UK, or for example the euro's track in Bilbao.
That is my theory
And if this is wrong for some scientific reason, I don't care, since I tried it numerous times on track, and that is simply how they behave