Katadori ikkyō irimi

The starting position is omote - omote, i.e. Uke facing Nage. Nage tries an Irimi from Ura. Then Nage's Omote meets Uke's Ura. From a technical point of view, this is very elementary. It's all about logic here. Nage guides Uke's arm so that his Omote meets Uke's Ura.
The forced guiding of Uke's the elbow corresponds to a simple mathematics. Just guiding the hand of Uke has almost no effect. If you fix Uke's hand instead and try to move his elbow, Uke can easily brace himself against it.
So a different math is needed. The elbow must be brought towards the shoulder so that there is a connection between Uke's hand and his shoulder. Maintaining this connection Uke's arm is moved. This is ikkyō.

Katadori ikkyō irimi

Starting position is again omote - omote. If Uke's arm is bent instead of straight, only Nikyō is possible. Ikkyō is possible from Ura.
With Uke's fist closed, Nage applies the crocodile grip to guide Nage's arm and create the situation Omote vs Ura.
Uke does normally not bend his wrist. But Nage pretends to bend, which compresses the forearm. When Uke is on the ground, his wrist also bends. There are therefore only two situations. Either Uke's wrist bends or he has to step forward. If the wrist doesn't bend, the whole arm goes forward until the wrist finally bends.

Osamu Obata (CAF/AIKIKAI)

Shomen uchi ikkyō