21. 12 inspections. There are three surfaces on a knife’s blade that require inspection: the sharp edge in the middle and the two sides below it [pictured is a cross section of a knife with the three surfaces at the top]. Each of the surfaces must be checked twice – once in each direction. In other words, 6 passes are done with the flesh of the finger and 6 with the nail, for a total of 12 passes. There is a hint to this in the posuk “בזה ושחטתם ” (שמואל א' י"ד, ל"ד ) – “בזה ” in gematria equals 12 (שמלה חדשה סק"ז ).
22. Today. Since we only check the knife’s blade with the nail and not the flesh, we are short 6 passes (19). Some do 12 passes with the nail to fulfill the minhag of 12 inspections (דעת קדושים סק"ט ). 6 inspections should be done with the nail, section by section, with a slow motion, and along the entire length of the knife. Afterward, another 6 inspections should be done with the nail with a quicker motion along the entire knife at once.
