An engine flush is a process of adding a chemical cleaner to the engine oil and then running the engine for a short period to remove built-up deposits and sludge from inside the engine. While an engine flush can help clean some of the engine's internal components, including the oil passages, it is unlikely to clean the pistons.
The pistons are located in the combustion chamber of the engine, and they are constantly exposed to high temperatures, pressure, and combustion gases. Over time, carbon deposits can build up on the piston heads and inside the ring grooves, which can cause problems such as reduced power, increased fuel consumption, and even engine damage.
While an engine flush may help to remove some of the carbon buildup inside the engine, it is unlikely to completely clean the pistons. To clean the pistons, a more thorough method such as disassembling the engine and manually cleaning the components would be required.
If you're experiencing problems related to carbon buildup on your pistons, it's best to consult with a qualified mechanic who can diagnose the problem and recommend an appropriate solution.