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.