From the following site:
The Lacrosse Forums - Stick Tech FAQ (read before posting) Pinch
This just means how wide or narrow the face of the head is constructed. Some people like heads that are very wide, some like heads where the ball can barely fit through it.
A wider head, is simply easier for catching with. Wider heads also usually have a wider scooping area (atop the stick), making groundballs easier. The downfall of wider heads is that its harder to cradle with. Which is the main advantange of pinched heads. Players who feel comfortable catching passes can use a pinched, or narrow head. It takes less effort cradling and controlling the ball with a pinched head, simply because there's less space for the battle to rattle around in. As an attackman, I use a pretty pinched head (a finalizer), since I feel comfortable catching passes, and I like to have an upper hand in ball control.
In general, defensemen want a wider head to intercept passes with, attackmen want a pinched head for ball control, and middies used something in between. But hey, it varies from player to play. The Revo Pro is a very popular defensive head and its very pinched.