As you know, there are two types of brake bleeding. In one person method (as shown in the video below the text), he uses a bottle almost half-filled with fresh brake fluid with one end of the hose into it so that when he releases the brake pedal it is the FLUID which is sucked back into the brake lines not the AIR. This way, you can do it without any assistant. However, there is a big problem here. When he presses brake pedal, old and dirty fluid will enter the bottle and contaminates the whole fluid inside the bottle and when he releases brake pedal, that dirty fluid is sucked back into the system again!! So, what is the point of flushing the system when dirty fluid is constantly exiting and re-entering the system?! We need fresh new brake fluid to fill the system not the dirty one! Isn't it? It just doesn't make sense to me! Here is the video: https://www.youtube.com/watch?v=ag-Q_lP0dOk
Even good mechanics get a colleague to help so one pushes the brake and the other opens the bleed valve when the system is pressurized. Then the valve is shut just before all the pressure is dissipated. Most apprentices get shown how to do this correctly and then they get to use the pressure bleeding systems that allow one person to replenish the fluid in the complete system even if it has ABS.
Most of the newer pressure bleeders also record the amount of fluid used so it can be charged accurately - would not want to be overcharging clients would we?
Other things to note are points like some master cylinders are known to "flip" brake seals if the brake pedal is pushed completly to the floor - often putting a block under the pedal makes sure that the piston does not travel too far.
Another method is to use a spring loaded rod to push the brake pedal - then you can operate the valve as needed. Top up fluid, reload the spring rod and do the next one. Of course in the correct order but that is just obvious.
One other one-person method used was to remove the fluid reservoir and hang a reservoir with a long pipe from the top of the bonnet (hood for some) - the extra height was enough in many systems to get the fluid to flow sufficiently well to carry any bubbles along. Not always though. Clamping the pipe then putting the normal reservior back on needed care.
Along with what Solar Mike mentioned they make a brake bleed bottle that has a built in one way check valve to prevent air or old brake fluid from flowing back into the brake line allowing one person to bleed their brakes. Link below.