Each tooth alternates thickness, narrow wide narrow wide, all the way around the pulley. 

With an old school traditional pulley, all the teeth would be considered narrow with small differences in thickness between brands. 

When looking at your chain, you will see how each link is pinned together to the next and the gaps between chain links alternate from narrow to wide. 

The narrow wide pulley now mates seamlessly with the chain, almost locking the chain to the ring without much increase in drag.

Besides,as you ride over rugged terrain, your chain moves around quite a lot. 

The sole purpose of the narrow wide pulley is chain retention. 

The narrow wide tooth pattern keeps the chain from moving left to right and eventually popping off while riding. 

No one likes to drop their chain on the trail, especially in a race scenario where every second counts!

In addition ,narrow-wide helps to clean muds more efficiently.

It is also important for these narrow wide pulleys to be able to shed mud and grime to function properly.