The principal these work on is that the relatively small opening (compared to a large hood) creates a high velocity and sucks up the smoke and flame.
The body acts as an expansion chamber and funnels the hot air and smoke into the flue.
The fire pot sits immediately below and in front of the intake.
This leaves the fire open on three sides.
Sometimes the intake is a foot above the firepot.
If this style is used be sure to close the gap between the forge and the hood/flue.
A gap lets in cold air increasing the load on the flue while reducing the efficeincy at the same time.
Often these need a sheet of burning newsprint tossed in to start the draft just before starting the forge so that your shop doesn't get smoked up.
A small flap, hinged or removable extension "hood" can also increase the efficeincy for when the fire is fresh or just smouldering.
NOTES: The stack diameter can be reduced to a minimum of 10" but then the intake opening should be reduced accordingly.
These have also been built with a long horizontal stack (round or square) before turning vertical.
This gets the stack away from the side of the building and does not need a roof penetration.