At the keyboard, once a finger has struck a key, there's nothing more that can be done with that finger to influence the sound of the note other then to control the note's duration by keeping the key depressed a shorter or longer period of time. Of course, the pedals may also be used to control the note's duration, but that's irrelevant to my study of Bach's Two Part Inventions because nowhere do I use the pedals while playing these 18th century pieces. Anyway, the pedals are beside the point.
While a greater or lesser amount of force may be used to strike the key depending on the desired volume of the note, the amount of force required to keep the key depressed is always the same and is always less than the force required to strike the key sufficiently hard to produce a sound.
In reviewing Invention #9, a relatively slower Invention, I became conscious that, particularly in my left hand, I was sometimes keeping the key depressed with about the same force that I had used to strike the key. This is a very fundamental flaw in keyboard technique that, especially in louder passages, results in a great deal of wasted energy and tension in the hand. I thought I had wrung that problem out of my playing, but now I find that I still have some work to do.
I find that the problem seems to be related to the emotional force with which I'm playing. If I'm strongly feeling the loudness of the playing, the emotion overshadows my technique and I tend to express that feeling through constant tension and force in my hands. The feeling is fine, but I need to learn to express it purely through the sound of the music and not in any way through extraneous tension in my technique.
So there's that.