Speed Bleeders® are a special "one way" fitting that can be installed on each brake caliper in place of the factory brake fluid bleed fitting. They enable a person to change the fluid as a "one person" operation since the bleeders have a one-way valve built into them. When loosened the fittings allow fluid out but air can not get into the brake lines. This way air can be bled from the brake lines by one person.
That makes sense... thanks Bill. Like a check valve in a line which allows one-way flow only. As long as the flow is against the valve, it remains open, but if the pump shuts down and flow starts to reverse, the valve seats and stops backflow.