The defining functional difference is that a FH is designed to draw air INTO the forward moving stream of combustion gasses. A FH will have an open front and sometimes an internal Venturi.
A flash hider will have a constricted front with an exit aperture typically only a bit larger than the size of the bullet. The goal here is for pressure to build up behind the bullet in the device which forces gas OUT.
For example. This is an A2 style flash hider. The interior is shaped like a Venturi.
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This is an A2 styled brake. It has an end cap designed to build up pressure behind the bullet and force gas out of the side cuts.
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