[1.II.9] Chord Attack
Suppose that you want to play the (LH) "do-so-mi-so" quadruplet ("Alberti accompaniment") many times in succession, very fast (as in the 3rd movement of Beethoven's Moonlight Sonata). The sequence you practice is CGEGC, where the last C is the conjunction. Since the conjunction is the same as the first note, you can "cycle" this quadruplet indefinitely without stopping. If you practice the quadruplet slowly and gradually speed it up HS, you will hit a "speed wall", a speed beyond which everything breaks down and stress builds up. The way to break this speed wall is to play the quadruplet as a single chord (CEG). You have gone from slow speed to infinite speed! This is called a chord attack. Now you only have to learn to slow down, which is easier than speeding up because there is no speed wall when you are slowing down. The key is -- how do you slow down?
First play the chord and bounce the hand up and down at the frequency at which the quadruplet will be repeated (say, between one and two times a second); this teaches the hand, wrist, arms, shoulder, etc., what they need to do for fast repetitions, and to exercise the appropriate muscles. Note that the fingers are now positioned correctly for fast playing; they are resting comfortably on the keys and slightly curled. Slow down and speed up the bounce frequency (even beyond the required speed!), noting how to alter the wrist, arm, fingers, etc., positions and motions to maximize comfort and avoid fatigue. If you feel fatigue after a while, then you are either doing something wrong, or else you have not yet acquired the technique of bouncing the chords. You will need to practice it until you can do that without tiring because if you can't do it for a chord, you will never do it for quadruplets.
Keep the fingers close to or on the keys as you increase speed. Get your whole body involved; shoulders, upper and lower arms, wrist. The sensation is to play from your shoulders and arms, not your fingertips. When you can play this softly, relaxed, fast, and without any feeling of fatigue, you have made progress. Make sure that you are playing perfect chords (all notes landing at the same time) because, without this kind of sensitivity, you will not have the accuracy to play fast. It is important to practice the slow bounce because that is when you can work on the accuracy and relaxation. Accuracy improves faster at the slower speeds. However, it is absolutely essential that you get up to fast speeds (if only briefly) before slowing down. When you slow down, try to maintain the same motions that were required at high speed, because that is what you need to ultimately practice.