A ballhead... Imagine you're holding a Christmas ornament in your hand. If you hold your hand loosely the ornament can move freely in just about any direction, until the hanger hook thingee hits your thumb or fingers. That's a lot of free movement. But squeeze your hand and the ornament is stuck in place, and doesn't move.
This is how a ballhead works. Now imagine that the camera would attach to the part of the ornament where you attach the hanger. Again, with the grip loosened the camera can move freely, but the camera plate will hit the edge of the gripping portion of the head so you are somewhat limited in movement. It's also easy to spin, which makes panning easy. You can quickly and easily point the camera in just about any direction.
With a 3-way head you can also turn the camera in almost any direction, but it has 3 locks which will allow movement in 1 direction at a time. So you loosen 1 lock, and you can point the camera up or down. Loosen another and you can tilt the camera left or right. Loosen the 3rd lock and now you can pan the camera. Again, these locks are generally loosened and tightened one at a time. This means that it takes time to swing to the left and raise the camera a little. By the time you do the car or bird or deer will be gone.
A 3-way is preferred for landscapes and such, basically because mountains and tress don't move so you have plenty of time to move the camera to the position you wish.
From these explanations it seems like a ball-head is the best of both worlds, but ball-heads have a very annoying problem in that they usually creep. That is, you loosen the ball, position the camera and tighten the ball - but when you let go the camera will almost always move a little due to the weight of the camera and lens. Its very unusual to be able to balance a camera perfectly over a head, so it will be heavy on one side. When you let go, the ball-head will slip a little, and gravity moves things a little.
This is not always a problem if you're not so finicky about framing. I am finicky, and when I set up a scene in a viewfinder I want it to stay that way! If the front-heavy camera has a tendency to flop forward a degree or two it may destroy all my efforts to perfectly frame the scene. So I don't use a ball-head when doing landscapes or similar.
The general rule is to use a ball-head for action, a 3-way for landscapes. However, a ball-head is more versatile and *can* be used for both, whereas a 3-way would be darned difficult to use when trying to shoot cars at a racetrack, or to track a bird in flight. A ball-head might slip a bit when doing landscapes, but this is greatly lessened with short/light lenses, and you can get used to it. If you find that a ball-head doesn't cut it for both then get a 3-way later.
In a nutshell, CPALIU, you will definitely want a ballhead for motorsports.
But this brings up another thing - tripods aren't a best best at a racetrack! Even with a ball-head there will be times when the tripod is in your way. The 100-400 isn't heavy enough to *require* a tripod, but it's not light either. I'd strongly suggest that you look into a monopod with a tilt/swivel head. Something like this
- or even better, get one with a tilt/swivel with a quick-release plate, like this
.
A tilt/swivel head basically allows you to point the camera up or down. You tilt the lens by tilting the monopod, and pan by spinning the monopod. For the 100-400 and motorsports this will be a LOT better than a tripod with a ballhead. And it's much lighter and a lot cheaper, too. The quick release plate will let you easily remove the camera from the monopod and use it handheld. For motorsports you'll do this a lot with the 100-400. Usually, probably, until you get tired and use the monopod to give your muscles a break.