If you want to leave corner pins, I have found a sure fire way to do it.
Tell yourself that you absolutely NEED this strike. You have the front 10, or you have to have it to win the US Open, or take the game, or win $10000, or something else. I guarantee that you WILL leave corner pins.
Seriously - you don't have to leave a spare to shoot the spare. You can do it at a full rack. Multi wood spares, no. But single pins, yes.
I constantly do spare drills when I practice. But I wouldn't throw away league fees on this.
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Ability is what you're capable of doing.
Motivation determines what you do.
Attitude determines how well you do it.
- Lou Holtz