These guys say:
"Gasoline engines achieve maximum efficiency when operated at the knock limit. Knock control ignition systems enable an engine to operate in either continuous or intermittent light knock...
Experience with knock control engines in passenger cars has shown erosion damage on pistons...
Nickel coating has been developed as an effective and reliable technique to protect pistons from combustion knock erosion.
Additional benefits of nickel coated pistons include:- Reduced piston deposits
- Increased wear resistance in the top ring groove
- Reduced cylinder head temperatures"
https://www.sae.org/publications/tec...ontent/900453/
These guys add "Heat reflection" to the list:
https://auto.jepistons.com/blog/what...ng-for-pistons
I DON'T 'see' the "Reduced cylinder head temperatures" unless the head (and valve bottoms) is also coated and reflecting IR etc light/heat back into the chamber..?
The pics also don't seem to show any piston top polishing to increase reflection and reduce heat absorbing surface area.
So perhaps the claims are made based on Nickel's low thermal conductivity of 90 W/mK, vs 250 W/mK for Aluminium.
BUT
at a thickness of 25 to 100 microns..??!
Now I'm going to take this straight to the Unicorn Corral again! 
Steam Reformation is the process of reacting steam with a hydrocarbon (fuel) to form Hydrogen and CO (burnable) as a by-product.
Nickel is the catalyst used.
(The process generally happens at the temperatures and pressures found in diesel engines rather than gasoline engines, but bear with me here)
Perhaps just maybe a very thin and diffuse layer of hydrogen forms on the piston surface and if/when knock starts; said layer causes the flame to spread out over a larger area, decreasing the corrosive etc effects..?
P=F/A after all.
And the flame speed of H2 is WAY higher than that of HCs.
Lets take this idea further:
Lets say the whole head and the valves are also coated with Nickel and you have a 'Nickel' Spark Plug.
(Perhaps even the cylinder walls: Nikasil...
Lets also say you also have a some steam or water mist injection going on.
H2O and HC is in contact with the hot intake tract and valve during the compression, power and exhaust stroke and even more so during intake! (much flow).
Then in the cylinder during intake and compression and even the power stroke:
During compression temperatures and pressure (amount of gasses in contact with surfaces) goes up.
Even more so during the power stroke.
Also keep in mind that the flame front dies out about 2.5mm before the cylinder wall, leaving a 2.5mm thick 'pipe' of 'unburned' air/fuel.
Less for pistons and exhaust valve as they're hotter, but still.
Now if there's a slight increase in the amount of H2 close to these surfaces..?
(See Quench Distance)
Having a bit more H2 in the 'Nickel Spark Plug' region would get the fires started a bit faster too, which relates into more pressure at usable crank angles and slightly delayed ignition timing, reducing losses.
Then there's the reflected light/IR/heat during combustion.
That helps on it's own, but water is known to dissociate a bit better in IR light too.
Even without a Nickel surface:
Thermolosis:
The decomposition of water into H2 (and OH, O3, etc type radicals) happens from around 2200 C, which is below the temperature of combustion..!
Sooo...
What are the chances that one could mod one's engine unto a slight Syngas etc factory, to produce a tiny but useful bit of H2..???
Especially in Diesel engines.
And what else could one do to increase the effect, of any???
One last thought:
Those HHO cells tend to be hot, steamy things.
Perhaps the steam is turned into Syngas etc more than even the HHO nuts realise..?
They do seem more effective in Diesels where pressures and compression temperatures are higher...