[1.II.17] Importance of Slow Play
Having pointed out the dangers of slow play, we now discuss why slow play is indispensable. Always end a practice session by playing slowly at least once. This is the most important rule for good PPI. You should also cultivate a habit of doing this when switching hands during HS practice; before switching, play slowly at least once. This may be one of the most important rules of this chapter because it has such an inordinately large effect on technique improvement. It is beneficial to both the immediate improvement and to PPI. One reason why it works may be that you can completely relax (see section II.14). Another reason may be that you tend to pick up more bad habits than you realize while playing fast, and you can "erase" these habits with slow play. Contrary to intuition, playing slowly without mistakes is difficult (until you have completely mastered the passage). Thus slow play is a good way to test whether you have really learned this piece of music.
The effect of a final slow play on PPI is so dramatic that you can easily demonstrate it for yourself. Try one practice session in which you only play fast and see what happens the next day. Then try one in which you play slowly before quitting, and see what happens on the next day. Or you can practice a passage fast only and another passage (of the same difficulty) slowly at the end and compare them the next day. This effect is cumulative, so that if you were to repeat this experiment with the same two passages for a long time, you will eventually find a huge difference in the way you can play them.
How slow is slow? That is a judgment call, and depends on your skill level. Below a certain speed, the slow play will lose its beneficial effects. It is important, when playing slowly, to maintain the same motion as when playing fast. If you play too slowly, this may become impossible. Also, playing too slowly will take too long, wasting time. The best speed to try first is one at which you can play as accurately as you want, around 1/2 to 3/4 speed. Slow play is also needed for memorizing (see section III.6.8). The optimum slow speed for memorizing, below about 1/2 speed, is slower than that needed for PPI conditioning. As technique improves, this slow speed can become faster. Some famous pianists have been observed to practice very slowly! Some accounts document practice at one note per second, which sounds almost irrational, but may benefit musicality.
An important skill to practice when playing slowly is to think ahead of the music. When practicing a new piece fast, there is a tendency to mentally fall behind the music and this can become a habit. This is bad because that is how you lose control. Think ahead when playing slowly and then try to maintain that lead when you get back up to speed. By thinking ahead, you can usually foresee flubs or difficulties coming and have the time to take appropriate action.