So far, this has been my (very limited) experience as well. Interestingly, the better I learn to respond to my sparring partners at first rather un-committed attacks, the more committed he's been forced to become. His attacks are turning more and more into something that I could really use for an aikido technique.
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What is stopping me at this point isn't really his manner of atttacking anymore, it's that the mere idea of executing a "technique" makes me stiffen my arms and that gives him a chance to use them as levers...
This observation in turn has made me look more into what I do in our regular training - and of course the same tension is there as well, but because it's very slight, it's easy to ignore, intentionally or not, in the more controlled circumstances of our forms training. Uke isn't free to exploit it either, which means it's up to me to be honest with myself in that situation.
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We played a bit with really committed shomenuchi last Monday in class, btw. It both made the technique of the night (iriminage) easier to do, and more difficult. Easier because there was something to do it with, so to say, but more difficult because tori really had to move right away. Lot's of people getting very high with their centers, into bad positions, pushing and pulling, blocking the attack. At the same time, there was a very clear sensation of the whole group getting more focused, and more relaxed or free as well, despite the anxiety of the startling attacks.