Here's another tip...
Two pyros are facing each other from maximum flamethrower range and flaming, one moves backwards in a straight line, the other moves towards them in a straight line. They both fire at the same time and keep up a steady stream. Who dies first?
The answer: The one moving forwards.
Why? Because while the one moving forwards can hit the other, they only do minimal damage due to the hitboxes touching at maximum range. Their hitboxes are also spread out more, so fewer hit.
The one moving backwards hits the other with hitboxes that hit at what counts as a closer range. Also, with the momentum, the hitboxes are bunched up in a smaller area. So more hit at a greater damage, sometimes 2-3 times as much.