The key bed is the frame underneath the piano keyboard. When you depress a key you move its hammer towards the strings. You want to complete the key movement all the way to the bottom so that you can control the swing of the hammer to the strings, but once you reach the bottom you want to quit!
The problems start if you keep pressing the key after it has stopped going down. If you continue to press, you are delivering power into the key bed. However, the key isn't moving anymore! The hammer has already hit the string, and you aren't having any effect on your sound. Worse, you are inhibiting your ability to have a positive effect on the sound you wish to produce next.
Pressing into the key bed takes time, releasing that pressure takes time, and you don't have that much time if you are playing passage work in even a modest tempo. That is one way key bedding destroys your mobility.
Pressing into the key bed connects your arm to it. That connection is still active while your next finger moves the next key, so it robs your playing movement of the arm connection it could have had if your last note wasn't still key bedding. Your tone production therefore has no arm leverage to help it. That is another way key bedding destroys your facility and that weakness of tone production also destroys your musicality.
Here is how to key bed in slow motion. Play a B major scale silently. Depress the first B slowly without producing a tone. Now dig into the bottom of the key. What do you hear? Nothing! The work at the bottom extreme of the key's descent does not help you. Keep digging. While maintaining the pressure on B, silently depress C-sharp. Now press hard on C-sharp and release your B. You still haven't produced a sound, but you're working like the Devil!
Continue in this fashion up the scale, pressing into the key bed of the last note while silently depressing the next one. Transfer the pressure to the next silent note. If you do this right, you will be sore and exhausted after completing your scale, and you will never have made a sound. Congratulations, you have mastered the ruination of your piano technique!
Are you playing this way more or less all the time? That would be alarming, wouldn't it? I hope this doesn't apply to you. Let's check. Ask yourself these questions. Do you get frustrated with pianos with "heavy" actions? Are they difficult to play? Do you work harder and harder and it doesn't seem ever to be quite enough? If you answered yes to any of these questions, you have been outed! You're a key bedder.
There are no heavy piano actions. There are some pianos that are a little different from what you expect, but they just are never really all that heavy. You are a little surprised by the unaccustomed sensation, and then you compensate by working a little harder. That's when you get into trouble. Once you dig into the keys just a little you have fallen into a trap. Your effort into the key bed can't help your sound, which is frustrating. Your instinct is to try harder, but no matter how hard you press, it doesn't help. That's the trap of key bedding.
If you are struggling with a recalcitrant piano action, try flopping unsupported, mushy fingers onto the keys. Don't play anything correctly. Just mash some keys with a completely inert hand. The keys go down fine, don't they? News flash: it is never any harder to play than that! The piano doesn't magically resist you when it detects you are doing something difficult.
There is only one place a piano can resist you and be hard to overcome, and that's at the key bed. If playing seems like hard work to you, you're key bedding.
A great way to get out of the habit of key bedding is to play slowly and allow your knuckles to rebound upwards at the conclusion of each attack. You don't want to block your arm into the bottom of the key. You don't want to "stand" on the key with your fingers braced rigidly. Instead, while moving to the bottom of the key for each attack, upon reaching the limit of the key's motion your knuckles should follow through upwards a bit. It prevents you from continuing to drive pressure into the key. This approach preserves a soft state for the hand. It remains pliable and unstrained, buying you mobility. In fact, while holding any sustained tones you must always be able to move your hands and fingers up and down and around a bit.
Relaxation can be an elusive ideal. If you habitually key bed, you are working too hard and impeding your mobility. In other words, you are tense. Simply trying to relax may not help you. An understanding of key bedding and how to cure it will. Then relaxation will come to you spontaneously as a side effect of correct application of effective technique.