Using the index finger of your left hand. Strike the fretboard (e.g., just behind 3rd fret of the A string, C note) using enough force so that a note is sounded. The note is not picked with the right hand. The sound is created only by the left hand finger hitting the string onto the fretboard. This is called a Hammer - Attack.
Any left hand finger can be used for a Hammer - Attack. Try the previous exercise using your second, third and little finger to sound the note.