Summary:
The method that trains the self-defence ability of Aikido is nothing but the severe rational training at usual practices without any collusion.
In Iwama style people train in severe Katai-keiko (Solid practice) first and then proceed to Ki-no-nagare (Flowing Practice). However in Aikikai style people neglect the training of Katai-keiko. Furthermore Aikidoka (Tori) and their partner (Uke) harmonize with each other in its Ki-no-nagare. This doesn't resolve the problem in which the opponent doesn't move as expected.
In Ki-no-nagare of Iwama style Tori would harmonize to Uke one-sidedly with his/her techniques. This is the true meaning of Awase (harmony) of Aikido.
In conclusion first we train Katai-keiko severely and second we harmonize to Uke one-sidedly in Ki-no-nagare — by these two practices our strength (self-defence ability) of Aikido would be trained.
Most sports have rules for matches. Several martial arts such as Judo, Kendo, Karate and so on are the same. In these cases the athletes basically try to train in order to win. Generally Aikido doesn't have any matches and rules for matches, so the trainees are apt to avoid hard training and to fall into a sense of self-satisfaction. This fact is true although the reason why Aikido doesn't have matches is another problem.
There are some small Aikido groups that allow matches and some leaders of these groups state "By competing in the matches, people will know their own skill and become modest not arrogant."
Although I strongly disagree with these kinds of matches, we must frankly analyze these words. We should always evaluate our skill with cool mind, discarding our sense of self-satisfaction. In order to do this we must thoroughly get rid of any cooperation or collusion. The ability to perform self-defence effectively in an actual conflict is never connected with how attractively we execute techniques at usual practices or at demonstrations.
The central part of Aikikai style is Ki-no-nagare, in which an Aikidoka and their partner are moving harmoniously from beginning to end. I wonder whether or not there is any cooperation or collusion.
Suppose that Aikidoka of Aikikai style whose strength are almost equal stand face to face and a one grabs the other firmly with both hands. Can the other Aikidoka brush the hands aside (break the grip)? Let's say that this can be done just before being grabbed firmly. However it seems certain that the Aikidoka would have some level of anxiety regarding whether or not they could effectively handle the scenario that entails their being grabbed firmly.
It is the same as the situation that the 2nd Doshu once stated as follows: "In Aikido it isn't necessary for you (Uke) to resist against your partner (Tori). By working with Tori's movement you will learn to gauge how much pressure and resistance to apply. This is effective training." — Best Aikido - Aikido Question and Answer - Page 18.
In short the 2nd Doshu tried to realize this thought on "Harmonization" into the practical aspect of Aikido and so adopted the method that both Tori and Uke harmonize with each other. Let's take this statement as a starting point. In an actual self-defence situation, if your technique go well, that is all right. However if the opponent tries to resist your doing by his martial force, for example, and doesn't move as you expect, don't you worry about the case?
Regrettably the 2nd Doshu's words don't offer any relief from this anxiety.
On the other hand at the first "Katai-keiko" stage of Iwama style, people experience that their techniques work every time even if their partner (Uke) grabs them firmly or strike them strongly on the head. After this they move to the "Ki-no-nagare" stage. Here it is not necessary that Tori and Uke harmonize with each other as Aikikai style and Uke doesn't have to "gauge" what amount of pressure Tori requires at all and there no cooperation/collusion.
At that time Tori always can confront Uke's free attacks with his/her self-confidence based on Katai-keiko and if only Tori(he/she) concentrates to harmonize to Uke's movement one-sidedly, his/her techniques would be executed against anyone at any time. I think that the leaders must consider this precaution not to cooperate between Uke and Tori at usual practices.
In other words the true meaning of Awase (harmony) of Aikido is that Tori relaxes without martial power, pours out his/her breath power and then 100% harmonizes to Uke's free attacks and always makes Uke quite cooperative by harmonizing one-sidedly. And this true Awase is the ultimate element that makes Aikido Aikido.
Needless to say various Atemis (face or body brows) are useful in this situation.
In conclusion first we should learn the detailed forms and order of each technique and the strong breath power at Katai-keiko. Second we concentrate to coordinate to our partners' movements one-sidedly (no cooperation from Uke) in Ki-no-nagare. This is the true Awase.
By these two practices our strength (self-defence ability) of Aikido would be trained and achieved.