Tenkanashi

Now we really start to pick up speed in tegatana (mentally - not physically). The previous two moves were pretty basic, and while this one's not really a monster, it is an altogether new thing. The third motion in the walking kata, tenkanashi, is a combination of two fundamental moves - nanameashi (the diagonal step) and a hip switch, which will be isolated later in the kata. Tenkanashi is also fundamentally different forward and backward, whereas the previous movements were the same regardless of the direction.
.

To do tenkanashi forward, you turn off a leg, fall diagonally forward to that side, and as that foot is landing turn the leg inward as close to 90 degrees as you can. The following recovery step happens as your body turns. Imagine evading a lunge and turning to face the attacker. As such, tenkanashi is the fundamental evasion used in aikido. Shomenashi and wakiashi automatically become tenkanashi whenever tori is presented with an attacker to center on.
.

Tenkanashi backwards breaks the rule of thumb established earlier in the exercise that whatever direction youre moving the closest foot moves first. To tenkanashi backwards, you relax the close foot and begin falling backwards (almost as if sitting down) then the opposite foot moves into place and the first foot becomes the recovery foot. Try it a few times moving the near foot first and you quickly find that nobody has the range of motion in their hip to inwardly rotate the front foot enough.
.

As the fundamental evasion in aikido, tenkanashi deserves a few minutes of paired practice in each class. Face uke at maai and as uke starts his lunge, tori slips out of the way using tenkanashi and puts hands up between his center (face) and uke's. When this exercise becomes commonplace, put a rubber knife in uke's hand and all of a sudden tori has a new perspective on the value of tenkanashi. Alternately, use the fast-looking lunge punches or lead jabs from karate as flavoring for this drill.