With yokomen attacks, whatever the technique, Saito Sensei explained that control by tori's rear forearm should never be achieved by going up under the attacking arm of aite.
Doing so on a powerful attack would be totally unrealistic, and a guarantee of receiving the yokomen in the face, as explained in the video (I saw a practitioner who made this mistake get knocked out):
Why can aite's yokomen penetrate and touch in such a case? Because tori's back arm, if it tries to pass underneath, must leave the kokyu position (the two arms in a circle in front of the chest) to make an independent movement, a movement of the arm. By leaving the kokyu position, the body can no longer move around the axis of rotation, and both power and safety are lost.
At the moment of irimi, tori's rear forearm must always control the attack of aite in the upper position, as can be seen on the photo, never in the lower position:
This forearm then acts as a shield on which uke's strike slides, being diverted from its trajectory and unable to reach its target.
