Basically it occurs when the shock heats up & bubbles start forming in the oil causing aeration. This is turn reduces the viscosity of the oil & allows it to pass through the holes in the piston faster, causing the shocks to go ‘soft’ & reduces their effectiveness.
Some shock manufacturers try to overcome this by charging the oil with nitrogen, others use a sealed foam cell, others argue that a well designed shock shouldn’t need either, & a whole host of others approaches.
Top of the range shocks tend to have an external reservoir to maintain the shocks performance (Bypass shocks).
By its nature, off-roading can be brutal on shocks. So whilst a shock could perform very well on the road, it could fade & perform very differently & poorly during heavy offroad use.
Dune driving is where I’ve experienced this the most. When it happens, I just stop for a while & let the shocks cool down. Gravel roads is another terrain where shocks can heat up pretty quickly, especially if your going a bit fast.
Out of all the shocks I’ve had, here is my rating of those shocks, from best to worst:
1.) Fox 2.0 Racing (incredible product)
2.) Zone Nitro (Impressive)
3.) Procomp (pretty good, but not the best)
4.) Old Man Emu (piss poor, especially for the price)
The Fox is an example of a clever design. One of differences is that the shock tube is made from aluminum, which is a better conductor of heat than steel. Another thing to note is that the shock body is 5mm thick. So both of these features presumably dissipate the heat much faster than crappy shocks. They pretty much won’t fade at all (I’ve tried).
Other top shocks like Koni & Bilstein also focus on proper design including accurate valving & correct oil viscosity.
Examples: Here are some interesting graphs that illustrate respective shock performances & how well they respond to heat & their susceptibility to fading.
There are 2 blue lines. The top line is offroad performance when they cold & the bottom blue line indicates their road performance when cold.
The top & bottom red lines indicate the drop off in performance in either offroad or on-road use.
OME - great for overlanding, but look at the drop when offroad.
RAW - pretty good. They are big shocks so I reckon the bigger volume of oil it houses takes longer to heat up.
TJM & Tough Dog - TD is clearly better
Bilstein & Koni - both show massive consistency, but Koni is the clear leader.
Content here
https://raw4x4.com.au/wp-content/upload ... omparo.pdf
. Then at full articulation, you can suspend the weight of the entire axle off them. 
Just FOXit 