Worked on this with Ben Sensei, Eric and Kanae yesterday.

Example: Using (Hidari hanmi to start)
Starting in Ai Hamni. Uke steps in with a chudan tsuki. Nage slides in and  blocks with left hand (block on elbow or higher) trapping uke‘s striking hand (right hand) to uke‘s body and an atemi with the right hand (simultaneously). The atemi hand then slides down uke‘s striking hand (this is useful if uke decides to retract his/her hand).
By trapping the hand initially this makes it difficult for uke to retract his/her hand. In any event even if uke does retract, once nage slides down the hand after the atemi, it becomes easy to grab uke‘s hand and perform the yonkyo.
Another key point is that nage should focus his/her energy on the inside knuckle of his/her index finger to apply pressure to uke‘s hand.