Sentence:  The most convenient pleximeter is the middle finger of the left hand, smoothly applied by its palmar surface on the part to be percussed. [...] The stroke is made, in most cases, with the tips of the two or three first fingers brought to a level, or with only a single finger. [...] Percussion may be strong or gentle; in the latter case, the stroke may be made by <WSD>filliping</WSD> gently upon the back of a single finger, or upon the nail. By gentle percussion, a sound is elicited whose character will depend on the condition of the wall, and of the parts immediately beneath it; whereas, when it is forcible, the deeper tissues will modify the result. 
Meaning of the word marked with <WSD> tags:  To strike or tap smartly.