Re: "Don't use strength!"
All of you might have been told " don't use strength" or "relax". You keep scratching your head and wonder helplessly since you ask yourself how to achieve such a state. I was in many many situations like this.
The thing is unless you can connect such arbitrary concepts as "strength" and "relaxation" to a physical concept which is real to you, then definitely it will take you decades to understand what "strength" and "relaxation" means or you might not get it no matter how you try.
IMHO, and I have validated this through my training, these arbitrary concepts are very related to the physics concept of "work". In physics, "work" is a totally different concept to how we view work. It is basically the effort to displace an object. Work has 3 components Work = force X displacement X cosine(anglebetween).
Imagine the simplest katatetori grab between nage and uke. Imagine also that uke is pushing or pulling nage's arm parallel to the floor for simplicity. Nage does maximum work (uses max strength) if he tries to push or pull along uke's same direction (parallel) of force (ki). Nage does not do work (therefore not use strength) if his motion is perpendicular to uke's arm/force. Here, the ki is the incoming force from uke and the angle is the angle between nage and uke's ki (force).
The thing is you see this in all the techniques if you just observe it in all grabs or strikes. It is a basic principle of aikido for me that I have discovered. The simplest way to imagine the body's lines of "ki"/force is to draw a stickman. Move perpendicular to uke's Ki line and you will not use "strength". This concept applies to the smallest part of the body which you can manipulate like the thumb and the largest parts like the spine.
Not using "strength" therefore to me is not doing "work" ie move perpendicular to uke's ki. "Relaxation" therefore is the least amount of "Energy", again another physics concept, expended. Energy is the capacity to do work. You must have energy to accomplish work, but since you're not doing any or minimally, you are therefore "relaxed". To do a certain amount of work, you must expend the same amount of energy.
You know you are doing work if any body part, especially the shoulders stiffen or you get locked/jammed in the technique.
These are very simple concepts you can start using in your next practices.