In a
four-stroke engine, the
valves are actuated by
camshafts. These are
eccentric, meaning that the
lobes are not perfectly round. They have rounded
protrusions, and as they turn, they cause the
valves to open. (
Valve springs return them to their closed positions, except in
desmodromic engines.) These protrusions generally lend an egg-shaped appearance to the cams.
Now, in a normal engine, you want a relatively gentle curve on the lobes, and a little overlap between the intake and exhaust cycles. This overlap gives the piston a little extra time to push out the exhaust gases, and the inrush of fresh air/fuel mixture helps as well.
In a high-performance racing engine, however, you want the valves to close quickly, so the declension after the peak of the lobe (as determined by the direction of rotation) is very steep, or "hot." You can tell when a car has really hot cams because of the uneven plat-plat-plat-plat-plat sputtering noises that come out of the tailpipe - the valves are literally slamming shut. This is very common in V8 engines. A similar sound can also be heard from certain motorcycle engines, especially Harley-Davidson choppers.