interesting question, indeed. Ikkyo is a very versatile and flexible technique, which exposes all the basic principles of Aikido pretty well.
My Aikido is basically Aikikai, strongly influenced by Senseis Frank Noel and Bernard Boirie, in France, and by Sensei Christian Tissier to an extent, through some of the top instructors of Aikido in Spain.
I have too been taught a lot of several different ikkyo. The one I prefer the most, as it works out very well with my being short and stout, could be described as a shomen uchi or a kesa giri on uke's attack.
As the wrists touch, the hand starts cutting down and slightly to the outside while I perform a deep irimi movement. This bends uke's arm enough for my inner hand to contact it, and again it's a cut slightly towards the exterior and deep down while I give a new step (it's a sankaku movement, like irimi nage would be, thus the triangle in the post icon), while the outer hand starts to move in a smooth curve like a U. When the technique is finished, uke's wrist is at my hip level, uke's elbow is tightly locked with tegatana and lower (this helped get uke's shoulder lower and keeps him from trying to get up easily, it works like a level).
(from certain point of view, this ikkyo is similar to kokyunage/sumi otoshi performed by the exterior of the arm/elbow instead of the inner side. The feeling is the same, you touch and cut, ka-boom!)
The pinnning down (or the projection, depending on the situation: I would run like a crazy from kansetsu waza in a randori, preferring to throw them away, preferrably towards one of the other uke's way. I value mobility
) it's done by stepping diagonally to the outside, keeping the lever activated. At the end, uke's arm is some 100/110 degrees from the spine (over 90 and below 160, or it would hardly work), the inner knee deep into his armpit (very important), and both tegatana controlling elbow (essential) and wrist (not as important, but equally useful, specially when getting up and away from uke). A variation of this is putting the outer tegatana on his elbow and using your inner arm to lock his other arm, or one of his legs (in the case uke is a very flexible kind of uke, this may be necessary to avoid a surprise punch or kick, you'd be amazed with one of my former partners, he was like chewgum...)
Anyway, other good forms I've been taught, imply entering into the attack and deviating upwards, while taking uke's whole axis, then it's a matter of stepping and taking him down with you, but this hardly works with very tall or very short ukes, and requires a very good timing, and careful attention to the feet, or you''ll step on uke's toes (or viceversa) and it's very painful.
hope this helps,
David S. de Lis