If racing is your thing, or just an automotive enthusiast, then you are most likely not afraid of swapping camshafts in your engine. However, the trick to making your car faster is not just replacing the cam, but replacing it with the "right" camshaft. This is where an understanding of valve timing is important. To help you better understand this, we will cover three basic terms used in evaluating the performance of valve timing.
On the power stroke, the combustion pushes the piston down in the cylinder. During this stroke, it is necessary to open the exhaust valve before the piston gets to the bottom of the cylinder. This will allow the excess pressure in the cylinder to "vent out" just before the piston reaches the bottom of the stroke. The term "Blow Down" is used to describe this event. Timing the exhaust valve in this manner assures no pressure is left in the cylinder to push against the piston on the exhaust stroke. Otherwise, there could be 20psi (or so) pushing against the piston as it starts up the cylinder. This would require some of your engine's power just to push the exhaust out of the cylinder! High RPM engines need to have the exhaust valve open sooner so the pressure has a better chance to exit the cylinder. However, at lower RPM's, opening the exhaust valve too soon means you didn't take full advantage of the power stroke.
As the engine cycles, there is a period when both the intake and exhaust valves are open at the same time. This valve timing is known as "overlap." Think of this as the exhaust and intake cycles overlapping each other. The valves are timed so the intake valve opens slightly before the piston reaches top dead center (TDC) on the exhaust stroke. Likewise, the exhaust valve is timed too close just after the piston starts down on the intake stroke. The objective of overlap is for the exhaust gas, which is already running down the exhaust pipe, to create an effect like a siphon and pull a fresh mixture into the combustion chamber. Otherwise, a small amount of burned gasses would remain in the combustion chamber and dilute the incoming mixture on the intake stroke. This valve timing is a product of the cam's duration and separation spec's. The science involved with overlap is quite complex. Pressures, runner lengths, temperature, and many other aspects influence how well the overlap effect works.
When the piston reaches the bottom of the cylinder on the intake stroke, the intake valve doesn't immediately close at this point. The intake valve remains open even though the piston is starting up the cylinder on the compression stroke. The expression "ram effect" is used to describe this event. Timing the intake valve in this manner allows an additional amount of fresh mixture to be rammed into the cylinder. The effect is very similar to water hammer plumbing. What happens is that during the intake stroke, the fresh mixture is running fast enough down the intake manifold and into the cylinder, that it cannot instantly stop when the piston stops at the bottom of the intake stroke. Just like the water hammer effect, the incoming mixture is rammed into the cylinder even though the piston may be starting up on the compression stroke. High RPM engines can have the intake valve remain open longer to take advantage of this ram effect. However, at low RPM's, the ram effect is not strong enough, and the piston will start to push the fresh mixture back out of the cylinder. Of all the different valve timing effects, this one can have the greatest impact on your engine's performance.