Shomenashi or nanameashi

The theoretical core of aikido is avoidance of force and the tactical skill that makes that possible is taisabaki (body displacement). The first several motions in the first kata in the system are simple taisabaki, in which the student visualizes an opponent approaching and steps off the line of attack. The first of these motions (shomenashi) is interesting because originally it was done as an attacking motion moving straight-forward from the starting position (hence the name - shomen means forward), but now it is done as a diagonal evasion offline, so actually the name of the motion ought to be nanameashi (diagonal step).
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An interesting thing appears when you break down this motion slowly. If you stand with your feet side by side on a line and try (slowly) to fall straight forward with your center then catch yourself by putting one of your feet under you, you'll find that it is impossible to fall straight forward (90 degrees to the starting line). You will always get a little bit of sideward motion toward the leg that is moving. If you move a little faster then it is possible to fool yourself into thinking that it is possible to move straight forward, but if you slow back down and look at it carefully you will see that the only way that you can move straight forward is to pre-load the off foot - i.e. to make two motions. It is important to differentiate stepping from falling because falling is more efficient and faster and more powerful than stepping. So in this way, the embusen (kata performance line) serves as a check as to whether you are stepping or falling. if you end up moving straight forward then you're stepping instead of falling.
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A similar thing happens in the first turn (and all the other turns) in the kata. It turns out (pun intended) that you cannot turn 180 degrees and end up on the same line you started on. If so, you are stepping (pirouetting) instead of falling. So, when doing the any of the 180 degree turns, watch to see that you end up off the line you started on.