Re: To block or not to block
Yes, I would agree with what you are saying Ron.
Most folks that adopt this yang/yang timing do not clash with the strike. As the first post stated, many folks just let the arm go by, etc. Most folks fall somewhere in between just letting the arm go by and/or having uke stop the downward motion of their strike at the point where it makes contact with nage's arm. Somewhere in the middle of that, we see the various parries and angles of deflection that many posters already mentioned. So, even in yang/yang applications, an "arm against arm" crash is noted to be "incorrect." I agree. However, I was trying to talk more from the point of view of tactical viability -- not so much ideals.
I think we can say that uke halting his/her strike is a violation of providing a realistic attack, so any kind of timing and/or angle of deflection that becomes dependent upon that halting cannot be considered tactically viable as an application of Aiki. Of course, letting the strike go by in a yang/yang application is not a crash of arms. Such a tactic can deal with realistic Shomenuchi - that is to say, uke can throw a realistic Shomenuchi and not have it injure nage's arm, or disturb his/her balance because of stressing nage's base of support, etc.
However, I was not only addressing this type of clash. I had mentioned two other elements that we should be aware of when employing yang/yang applications. One element addressed being too late to find shikaku (for this waza), and the other one addressed the clash that could come about in certain types of nage/uke pairings wherein nage employs a pulling energy to bring uke back into range for the prescribed kuzushi. These two elements are the more important characteristics of yang/yang applications that we should be aware of. This is true for me whether one is strong enough or not to crash with an uke's strike (as in blocking), etc. In addition, whether one lets the arm go by or not (or parries it or not), yang/yang timings force one have to confront these other two issues if one is still interested in the tactical application of Aiki. At least my experience has led me to this conclusion. A conclusion I have reached because of the experiments I had mentioned earlier.
If nage attempts to enter during the yang phase of the strike, he/she will only reach the back of an attacker that has stalled his/her attack for such purposes. Or maybe I should say, if nage attempts to enter during the yang phase of the strike, he/she will only reach the back of an attacker that does not intend to follow through and/or follow up in his/her initial Angle of Attack. As for an attacker that has full intention to follow through and/or follow up, said attacker will be too far away to be guided (and not pulled) into the spiraling kuzushi. This is a result of uke already having the capacity to place full weight upon his/her front leg before kuzushi takes place. Said leg can then be used to shift weight forward, to take another step forward, or to change direction. All of these things can be used by uke to move out of range - against the proper execution of the prescribed kuzushi (at the least). On the positive side, if one really wants to feel his/her own balance taken, have your nage try a yin/yang timing application and thus begin the kuzushi just as your weight is coming down on your front foot. Wow, what a ride!