One of our (Dojin Aikikai is the our in this case) former Aikido students who is now studying Karate with me was asking me about the usage of Tenchi-nage, and how he doesn't feel he ever really "got" the technique and didn't feel comfortable with it. In his case it would be the titled Ryote-tori (two hands grabbing two wrists from the front) Tenchi-nage (Heaven and Earth Throw) Irimi (into your opponent) (your transliteration may vary). Unfortunately for him, the school he was attending was shut down abruptly, but that's a different story, at a point in his training where he was expected to explore the technique beyond the basic mechanics of it.
Knowing that if one person has a question about something, there are probably more, and the fact that his question forced me to explore a technique I hadn't given a second thought to in 3+ years, I figured I'd do a little break down of the technique.
Now, for those unfamiliar with the art of Aikido as it exists outside of Steven Seagal movies, there are 2 training principles you need to know for this to make sense. One, Aikido is an amalgam martial art, you could call it a mixed martial art, but as that particular phrase is now almost exclusively associated with UFC style fighting, it presents an unrealistic image of what Aikido is to people, so I use amalgam. It is a blend of jujutsu (wrestling) with kenjustsu (sword fighting), yarijutsu (spear fighting), and kyudo (archery) movements and principles. The kyudo is what we are going to look at this time. Two, there are some Aikido techniques that exist not for their particular combat effectiveness, but as a teaching tool and stepping stone to future techniques. Ryote-tori Tenchi-nage Irimi is one of those "teaching techniques".
So, if you open your copy of Aikido and the Dynamic Sphere to page...wait, you don't keep a copy on you at all times? Ok, well, if you do find a copy, Projection #9, which begins on page 271 is Tenchi-nage Tenkan (around). The tenkan version is what would be combat applicable, and the irimi version which we are talking about leads into that one.
Before we go into the mechanics of the technique, we need to focus on what the technique is supposed to teach. The technique's main focus is to teach extension and projection with a lesser focus getting your hands to do two things at once. It's a good white belt technique because those the first two (and sometimes the third) are among the hardest concepts for the beginning Aikidoka (Eye-Kee-Doe-Ka)to get, that you have to extend through the target and the project them away.
To start with, both partners stand facing each other, at about half ma-ai (in Aikido, we generally just use one "fighting distance" if both partners extend both their arms straight out from the shoulders, making fists, and approach each other until their knuckles touch, this is ma-ai, also called So-ou Ma-ai. It is enough distance that you can take 1 step and strike your partner). The Uke ("fall guy" or the attacker) reaches out and grabs both the Nage's ("thrower" or the defender) wrists.
The nage then steps forward with one foot. The nage wants to just pass to the outside of the uke's foot and behind it, the hand on that side needs to reach down as if trying to pick up an object just beyond the point of their toe. At the same time, the opposite hand draws back to the nage's cheek/ear and rotates away from the body so that the palm is facing down. as you move forward, this hand will end up slightly behind the nage.
At this point, the uke needs to just stand there and try to hold on. The uke shouldn't move their feet (but if the first step is done right, the uke is often forced to step back with one foot to maintain balance and not fall) or let go if possible. Remember, we aren't concerned with combat effectiveness, we are concerned with learning principles. I know no one is going to just grab your wrists and stand there, but we aren't concerned with that now. What the uke needs to do is provide feed back to the nage. If the technique is correct so far, the uke should feel like they are starting to fall backwards, in some cases, with a nice deep extension of the downward hand and an uke whose back is not that flexible, they will even fall at this point, that's good. If the uke does not feel themselves falling or on the verge of falling, they need to let the nage know. It is a sign that the nage's first step and downward hand extension was not far enough behind the uke or went too wide, and they should return to the start position and begin again.
Once the uke's balance is broken, the nage steps forward with the other foot, bringing it towards and past the initial step, so that both feet end up on one side of the uke. The hand that is by the cheek extends forward and past the uke's ear, as if trying to grab an object off a far wall. At this point, the uke will be falling backwards, and would be advised to let go and execute a backfall or roll, if they haven't done so already.
Now, you say to me "Wait. I can't just walk forward, he's still there, I'll bump him, or worse, get kicked in the jimmy as he falls backwards!" Not true. If your second hand extends past the uke and you are really reaching, you will find that you have just accomplished "projection" and they will fall backward and away from you, and you have also just experienced another Aikido concept called "Body Displacement". Basically, physics tells us that two objects cannot occupy the same point in space/time simultaneously. Since the nage will now find their ending position to be just about where the uke was standing, this means the uke is moved out of the way. Body Displacement is a wonderful concept that comes into play in many martial arts techniques whether the practitioner understands it or not, but that's a different, and perhaps longer discussion.