A Nail constraint nails a single active rigid body to a position in the workspace. It works as if a solid bar connects the rigid body to the constraint position. You cannot use a Nail constraint on a passive rigid body.
If you animate an active rigid body with a field such as gravity, you can use a Nail constraint to create effects such as a ball swinging from a cord.
If you don’t turn on Set Initial Position, Maya puts the constraint at the rigid body’s center of mass.
To alter the constraint after creation, see Edit constraints.