The purpose of a "fly brake" is to lock up (or momentarily brake loose) the rear wheels of a car to assist it in turning hard when there's insufficient wheel lock (or space) to manage it with the steering wheel alone.
A fly brake is not for stopping a car and isn't for holding a parked car, but can be used in tandem with a mechanical handbrake if desired.

In essence, the cylinder used for the fly brake is a remote brake or clutch master cylinder. It has an inlet and an outlet, but instead of having a remote fluid reservoir feeding the inlet, it is simply plumbed, in-line, into the car's rear brake line and the inlet is fed from the car's regular master cylinder.

The single rear brake line (not suitable for diagonally split systems) from the car's master cylinder is cut and plumbed into the front port (yellow, above) in the handbrake cylinder and the rear section of the rear brake line (to the rear wheels) is then plumbed into the rear port (white, above).

A common .75" cylinder  is usually perfectly adequate. A clutch master cylinder from a Mk 2 or Mk 2a Land Rover would be ideal, though remote master cylinders are available from brake shops and retailers almost everywhere nowadays.  

In operation, the handbrake lever is pulled up and the piston within the fly brake cylinder immediately closes the line from the car's normal (pedal-operated) master cylinder, which prevents the master cylinder being pressurised.

Once the line to the master cylinder is closed off, the fly brake piston continues to move, applying pressure only to the rear brakes. 

The fly brake cylinder should be bled prior to bleeding the rear brakes by pressing on the brake pedal and pumping the fly brake to move any trapped air rearwards.