I would have to feel it ...
Oh yes. It is very difficult to do (mean: to explain, to "understand", to exchange) this with only words ... :-)
To get a hint maybe:
Make a fist with your left hand in front of your breast, arm horizontally, knuckles pointing to the right. Touch this fist "at the front" with you right palm. (like tori does with uke in the video). Now gently but clearly push the fist over to the left. Fist should give some resistance.
The feeling in fist and the palm is "a little bit similar" to the feeling in the hands (only then hands!) of tori and uke during the exercise shown in the video: To "press/push" someone with one's palm and being "pressed/pushed".
This does not show, what or how the body feels and how you "steer" uke using this atari. But maybe you might get an idea about the "construction" oft this way of contact: Instead of combine/blend my ki with uke's and give the new stream of those two ki another direction, here tori's ki goes directly to uke, meets/colides with his ki and redirect uke's
(!) ki. There is no merging of tori's and uke's, but tori kind of takes over uke / uke's ki and now tori directs uke's ki.
When I started aikidō I tried to get the timing and push a swing-door, when it was just opening, moving allready away from me. I added my push. I presume a lot of aikidō Beginners did things like this. But this feeling is clearly different from the feeling of atari like it was shown in the video.