How does an engine lose power due to valves?

"Frictional Losses
The camshaft, which opens and closes the valves, is driven by the crankshaft. The rotation of the camshaft and the movement of related components (rocker arms, valve springs) produce friction. This friction leads to energy losses, though they are relatively small compared to total engine losses. These friction losses are typically part of what’s known as mechanical efficiency losses and can amount to around 2-5% of the engine’s total power output."