Raising your hand only tells the professor that you, an individual, are confused. Interruptions by a single student that doesn't understand the material, when the rest of the class does, might actually cause the professor to focus too much attention on a subject that doesn't need it.

This is far better. There will always be a couple people that are not following. If the professor is getting 90% "I understood" responses, he can simply mention that anyone with questions on that topic is free to meet them after class, while the rest of the time can be spent on topics where there is a lower rate of understanding.