Blade could not keep his jaw from falling open for a second in sheer astonishment.

He could barely keep his jaw from swinging open.

It was all she could do to keep her jaw from dropping.

It took effort to keep his jaw from dropping.

Jack tried to keep his jaw from dropping, and half succeeded.

When he did, about a minute later, Bart tried to keep his jaw from falling open.

It took active effort to keep my jaw from dropping open.

They may be designed to keep your jaw in a more relaxed position or provide some other function.

Then Quark looked at the people in the room, and tried to keep his jaw from dropping.

He could see the effort she was making to keep her jaw in place.