If it is what I think you are talking about, many people used to let the birds enter the loft through an open door. The thought was that there would be absolutely no hesitance for the bird to fly right in after training or a race. They could then be caught and the countermark removed and clocked. This, or course, was before electronic clocking where the bird has to walk over the antenae pad to be clocked. You don't see this type of trapping much anymore as most people have gone to the efficiency of the electronic clocks.
open door trapping is for widowhood.the birds are allowed in and out for exercise.when they build their nests during mating,they will often fly in and out with nesting material from outside.this is done with an open door or window.also they will go out an fly and circle the loft clapping their wings only to come back down and then repeat the process.you know your birds are reacting to the system when they do this.i think they do this especially older flyers that have been on the system a few years,because the rest of the time they are in their boxes resting,usually with shades down so they dont even see outside.when you do turn them out they will fly and come in and out for hours.