Here are my ideas for playing octaves:
IDEA THAT DOESN"T SUCK #1:
1. Approach everything (including octaves) with a neutral hand and mechanism.
2. Play the note.
3. Recover with a neutral hand and mechanism.
Repeat.
Being in a state of relaxation at all times is key...EXCEPT for that split second when we actually activate the note. At that moment, the ends of our fingers and the structure of our hands firms up. I talk about this more next...
IDEA THAT DOESN"T SUCK #2:
As I've stated many times on this forum, I believe in playing with weight; our muscles should be used to aim the weight to the correct key.
However, weight alone is not enough.
Think "weight with structure." When we drop our hands on the key, the bridge of our hand (essentially our palms) must be firm enough to support the weight. It must not collapse. If it collapses, then we have just wasted energy and that will tire you out. I've have actually found that the firmer the bridge of my hand, the less tired I get when playing octaves.
IDEA THAT DOESN"T SUCK #3:
The idea of a balanced hand. This is such a simple idea, yet it fixes SO many problems, and not just octaves.
Imagine you are asked to play a C and a G. Now, look at the keys. What note is in the middle of those two pitches? "E" should be your answer. So, instead of thinking "I will play two notes with two fingers", think "I will balance my hand and drop my weight over the center of the pitches, which is the general area around "E".
Another example: How about an octave from C to C. What is the center point between those two notes? Somewhere around F and F#. So drop your weight there.
What about C and E? Balance your hand around D.
What about Ab and F? Balance your hand generally around C and C#.
So, instead of thinking two notes, think one drop. Think one drop aimed properly. If you had octaves in both hands (like the CHopin etude), think two drops instead of four notes.
Does this work for funky fingerings? Yes.
Example: How about finger 3 on C#/finger 5 on G. Look at the two notes. Where would the center of those pitches be? Somewhere around E. Drop your weight there.
This works for chords, octaves, and astoundingly confident pp chord playing. It takes the "hope" out of playing soft chords, and replaces it with a system that is reliable.
IDEA THAT DOESN"T SUCK #4:
Keep your torso, neck, and head stationary. Think "Stationary Torso." If you are flopping around with equal and opposite reactions to every arm thrust, then you are again wasting energy. Imagine a guy holding a jackhammer. He is keeping his body firm. What would happen if he relaxed his torso? The jackhammer would bounce out of control. A guy shooting a machine gun is another useful analogy.
IDEA THAT DOESN"T SUCK #5:
I believe in higher wrists for bravura octaves.
IDEA THAT DOESN"T SUCK #6:
Much emphasis is placed on wrist octaves, but there are also arm octaves. These involve stiffening the wrist and using your bigger levers. Practice playing scales and switching techniques from wrist to arm and back again WITHIN the same scale.
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To sum up, think:
Firm bridge and active fingertips
Weight with structure
Loose elbows
Balanced hands
Stationary torso
Higher wrist
Of course, there is a lot more to octaves. Start with these ideas.
Robert Henry