preselection actually doesn't happen in all countries, so I feel I have to break this chorus :)
In the Netherlands we have always had a system where every jury member had to see every image submitted (more or less). We learned that professional photographers actually are very well able to handle this kind of job, especially if they have some kind of editing function too. We learned that they can process actually thousands of images per hour - so if they are willing to spend a few hours and are helped by the right equipment (jury tool!) they can process a /lot/. For reference, the Netherlands had roughly 12k, 13k and 6k submissions in the past years. We asked the jury members to come up with their top-50/100, added those together and then discussed them during a real life jury meeting where the final decisions were made.
In other countries, this was undoable, and the list was split up in a way that each image was initially seen by one jury member (I believe this is the system used in Germany). They would then select a small number from their own stack, and together this would get a second round treatment.
Basically, what you see in all countries is that there are multiple rounds. First there is a round where, one way or another, the big stack is reduced to a much smaller number. Usually between 100 and 1000, depending on your starting set (Poland is probably an exception - they needed a round in between). Then, there us another round to reduce the set further to somewhere between 10 and 100 - which makes it comprehensible for the human brain. From this point on, you can start discussing the order the images have to be in.