A flush is where fresh oil is blown through all parts of the engine in order to flush out debris. It may be necessary to remove cams and separate conrods from the crankshaft to make sure that nothing gets stuck in the bearing surfaces.
You need to know all oil channels on the engine and the possible places where dirt can get trapped. Some devices like the cam timing advancers are tricky and may not be possible to disassemble. There are also some small filters in some parts of the engine - the notorious banjo filter.
Be aware that the debris is sharp and may perforate the oil filter and fingers if you aren't careful. Protective glasses is a good idea.
Last edited by ehsnils; 09-02-2013 at 02:12 AM..