Originally Posted by delloro

Pretty sure the gas pressure is to reduce foaming, and adds no appreciable rebound. Shocks need no pressure to extend, the springs do all of that.

ETA:

"When a twin tube shock is gas charged, the purpose is to minimize aeration of the hydraulic fluid, this occurs when the shock is working hard and heating up. The nitrogen gas pressure compresses air bubbles in the hydraulic fluid, which prevents oil and air mixing (foam). Without aeration, the shock is able to function more efficiently and reliably."

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"Foam Cell Twin Tube - A foam cell shock uses a compact foam sleeve inside the twin tube body, taking the place of a nitrogen gas charge. The foam cell is more compact than the nitrogen gas in terms of volume, leaving more room for hydraulic shock oil, which is the main cooling agent of the shock. Many foam cell shocks are developed for high heat environments, as they typically do better than Nitrogen Gas charged shocks in resisting fade through heavy use."

So it sounds like they are designed with heat in mind. I suppose should the foam get cooked, then aeration could follow, but I presume they selected foam to withstand the heat. I don't know why foam would cook before seals, o-rings, etc.

Shock Absorber Designs - Emulsion, Foam Cell, Nitrogen Gas, Monotube, – Shock Surplus