Grip the long sword with a rather floating feeling in your thumb and forefinger, with the middle finger neither tight nor slack, and with the last two fingers tight. It is bad to have play in your hands.
When you take up a sword, you must feel intent on cutting the enemy. As you cut an enemy you must not change your grip, and your hands must not "cower". When you dash the enemy's sword aside, or ward it off, or force it down, you must slightly change the feeling in your thumb and forefinger. Above all, you must be intent on cutting the enemy in the way you grip the sword.

Go Rin No Sho (1645) - The Water Book