Our muscles and tendons surrounding our joints contain position sensors - proprioceptors - that are constantly feeding our brain information. These sensors are the reason that you can touch the tip of your nose or clap your hands or scratch just the right spot on the back of your head without looking. As your joints move through space, they keep your brain apprised of where they are and what they are doing.
Except when they don't. Y'all have seen in movies the field sobriety test in which a police officer asks someone to stretch their arms out, tilt their head back, close their eyes, and touch the tips of their fingers to their nose. Well, there is another thing besides alcohol that can inhibit your proprioceptive sense of where your joints are and what they are doing - isometric muscular contraction.
When you contract the muscles on both sides of a joint to lock it into place, you essentially turn off the position sensors in that joint. Essentially you can think of it like this - if you will not allow your joints to move, then you will not allow the motion sensors in them to work.
One instructor told it like this - your muscles have two mutually exclusive modes - feeling or doing. When you are doing something with your muscles then you cannot use them to feel. When you are feeling things with your muscles, you cannot do (much) with them.
In judo and aikido we are forever preaching "relax,"and, "don't stiff-arm," and, "not so much upper body." etc.... This is because in our normal practice mode we don't want to brick our brain up in an impregnable fortress of muscle - you would rather be able to use your mind to tilt the odds in your favor and to do so, you have to give it a constant stream of input. If you turn off your inputs (by strengthing up) then you are being counter-productive.
You can also think of it as easier to learn how our bodies are interacting if you have light, pliable feelers on the other guy's body constantly feeding you information like a bug's antennae.
It's not (too) difficult to figure out how this works with your hands. What is really interesting is when your feet start acting like feelers or antennae during ashiwaza! When you start reaching out and checking what uke's doing with a foot instead of trying to kick uke out of the ground. Four antennae are better than two ;-)
Photo courtesy of aussiegall