This sign was placed on a wall in the Iwama Dojo by Saito Sensei to encourage correct practice.
It is often taking out of context. That Aikido is a passive light Art. It isn’t. It fact resistance is a part of Aikido training, but at some point students must have went through a phase of using too much resistance and thus Saito Sensei saw a need in putting up this sign. If you are stopping one technique from taking place you are not teaching it and have forgotten about the other 99% of the practice which calls into questions the motivation for training. If the desire to train is fueled by a deep curiosity to understand Aikido then the student will train in such a way as to best learn. Otherwise there is likely an urge to compete, which is often counterproductive to Aikido practice and also to most traditional Budo.
For example if I apply this to Kenjutsu training. Instead of using brute strength to overcome my partner I can use speed and easily confuse a student less experienced than myself. I have trained with people like this. It is completely unhelpful and requires little skill. The approach to study must be one that is effective and of help to fellow students.
In my own practice I see it as essential that each individual is able to progress as best they can. There is no point in grabbing a beginning student with all of your strength or a child the same way as an adult. Doing so would only encourage a tense and aggressive reaction devoid of technique. Another importance aspect is that using excessive strength always takes the student out of the moment. The Uke is not to preempt the technique but is too give a solid, honest grab and response. A link to a previous article on this subject is below.