Although this technique may look simple, slicing mushrooms is an advanced knife skill because it requires complete mastery over your guide hand technique.
When slicing mushrooms, always use the tip of your chef’s knife. This is where a well-balanced knife comes into play. A knife with good balance will allow you to move the tip of your knife up and down in a rapid, fluid motion.
I also like to change my grip to a modified two-finger grip with my index finger resting on top of the blade. Many people characterize this grip as poor technique, but I find that when used correctly, your blade follows the motion of your finger, giving you greater accuracy.
For the motion, move your wrist up and down, and remember to stay relaxed. If you tense your wrist, not only will it start to cramp, but it will also slow down your chopping speed. Your guide hand is there to guide your blade, as you rapidly move it up and down.
As long as your keep your knife grip relaxed and originate the blade movement from your wrist, you will astonish all your family and friends with the speed at which you can slice mushrooms.