It's very difficult for a technique to work under resistance (extreme or minimal)without some sort of kuzushi (whether physical or mental). In the case of Tenchi Nage I think 90% of that tech is timing and kuzushi. The rest is simply follow through because uke has no defence with no balance.
On the side of uke, he/she must learn to develop the sensitivity, flexibility and body control to react to the technique in the best and safest way possible, without succumbing to the need to "take a dive." This helps both parties understand how the technique affect body mechanics and exactly what twists, turns etc. make techniques effective.
On the reverse side, trying to predict the movement of techniques can be simply dangerous, as tori may switch without warning in the middle of a technique, expecially during resistance or spirited randori. Often I find myself aborting/disarming techniques to avoid damaging ukes who act in this way. Which of course lessens the utility of the practice to those involved.
Just my 5 cents.