When two-legged creatures walk they spend almost all their time in one of two conditions - either standing on the left leg and stepping to the right, or standing on the right leg and stepping to the left. It turns out that this makes a big difference in self-defense applications like in aikido. If we start evading exactly at ma-ai then we only have time for one body drop motion, so if we have to switch legs in order to go in our favored direction then we have to make two body drops and we increase our chances of getting hit. It is important to evade in the direction we are already going as uke passes ma-ai. So, if we are standing on the left leg falling to the right then we evade to the right and vice versa. This type of evasion is practiced in tegatana.
There is actually a third walking condition that happens that may not be as apparent as the first two. as we shift onto one leg our center is rising and we still have momentum toward that standing leg. In that case, it takes a lot of time and energy to stop and fall away from that standing side, so in this case we evade by continuing our rising step until we are "over the hill" and stepping down on the far side of the standing leg. This type of evasion is practiced in the beginning of hanasu.
So, it is important to practice falling naturally to the left or right as uke attacks but it is also important to practice evading by walking over the hill.