Hypereutectic pistons are cast with increased silicon content. Basically so much silicon is added to the aluminum that not all of it will be absorbed by the aluminum and therefore silicon particles are suspended in the aluminum forming a matrix. This improves the temperature limit of the piston, helps control the expansion characteristics of the piston, and has other benefits. To be honest I do not know all of the details. There are some downsides to hypereutectic pistons as well.
I've read that "Hypereutectic pistons will take a lot of pressure. You can stand a mountain on them and they won't break under the weight. However, the minute you ring them with the frequency that nitrous causes when it detonates, they shatter like glass. They don't melt; they break. Don't use them in a nitrous engine." Sports Compact Nitrous Injection
by Joe Pettitt
I'm not sure how they work in a non-nitrous engine.