Sorry I can't elaborate in detail on these items but I have heard of a few things.

Replace bushings and traditional style engine bearings with roller bearings.

Full roller valvetrain.

Eliminate valvetrain all together by using electric solenoid operated valves.

Hollow cams and cranks.

Hypereutectic pistons and coated piston skirts

Variable valvetrain timing.

"looser" machining tolerances.