Timothy Walters Kleinert
I get that the difference between semi-compliant and passive resistance is in the initial attack. I think I know what you're getting at. Semi-compliant would be those times the attacker does an "easy" attack or kinda "settles in" before nage responds? Passive resistance would be those times uke gives a "determined" or "realistic" attack? Is that right?
Yes and no. Realistic is a difficult word to use when talking about an attack in any kind of training below full-resistance freestyle randori because there is always some holding back of certain aspects of realism to allow the nage the chance to work on something he might not be able to do against a more realistic attack. Of course, which aspects of realism you choose to leave out and why is a major determinate of how useful your training will be against a truly realistic (or real) attack.
The difference between these two level really comes down to whether uke gives nage a connection to his center. In semi-compliant, uke exposes his center to nage's center through the connection and then lightly presses his center into nage in whatever direction is appropriate for the technique. In passive resistance, uke attacks in a way that doesn't expose his center.
Think of how a boxer (for example) would attack compared to the average aikido person. The boxer does not give his opponent a connection to his center (he also remains centered himself, but that's a a slightly different issue). However, a boxer or MMA fighter would also try to break connection if his opponent tried to establish it. This is at the active resistance level (or higher). In passive resistance uke does not give the connection, but does not try to break it either.
It's really just a training tool to work on the specific aspect of learning to establish connection with uke's center. That's one reason why I did not include in the original list. In practice, these levels are much more fluid, and ukemi could go from compliant to passive-resistance to active-resistance to countering all the same technique. If it still doesn't make sense at this point I wouldn't worry about it too much.