The mask punch and punch test are the first line of defense against accidents in fencing.

The Leon Paul mask punch uses a "tactile" indicator that it has reached 12kg.  The plunger hits the ball of your thumb, which tells you that you have reached the 12kg of force.

Instructions for Use:
Used partially from the Don Danulf's Academy of Defence web site at: http://www.musketeer.org/safety/mask-punch.html

Tactile Mask Punch
The most common type of mask punch available use a tactile indicator. It uses a plunger with a shaft length which "hits" the ball of your thumb when it reaches 12kg. Most are based on the published design in the FIE documents that was originally manufactured by Prieur. The TCA punch is nearly identical to the Prieur design but TCA recommends that it be used visually. In practice it can be used tactically if one takes the precautions described below where each punch tester is discussed.

To verify that you can accurately test masks with your mask punch using the tactile indicator method, test the punch on a scale, treating the scale like you would a mask. If it reaches 12kg (26.5lbs) when the indicator hits your thumb, then you can use it as a tactile punch. If you find that treating it as a visual indicating punch (see instructions below) yields accurate results, then use it visually. If neither method results in a 12kg test, you are either using the tool incorrectly or its spring is defective.

To use it, one holds the barrel of the punch tester in the hollow of the hand with the ball of your thumb over the end of the barrel.
Your other hand should be holding the mask, place your hand "behind" the area that you plan on testing, supporting the mesh with your fingers. (Do not put your hand "flat" behind the mask because the mask punch could hurt your hand if/when it pushes through the mesh.) Pull the mask into your lap.
Place the punch into the mesh where you want to test, do not put any pressure at this time, do not torque (no angular or rotary movements) the mesh in any way. Gently bend over the mask so that the punch ends up in the "hollow" between your shoulder and your chest. Ensure that the punch is at 90 degrees to the mesh, no pressure is on the mask or punch yet, and that you are not torquing the mask with the punch.
Relax your arm and shoulder.
Watch the punch and slowly press on the punch using your stomach muscles. This is very much like "bending over". I have done this in my lap while sitting, over a knee on a field, or on a table while sitting. (If the punch begins to torque or shake, immediately relax -- releasing pressure on the punch. Return to the previous step.)
STOP when you feel the end of the plunger hit the ball of your thumb or if the plunger deforms the mesh and the conical section slips through the mesh. Do not continue to push after the plunger hits your thumb. You can damage the mask this way. In theory the ball of your thumb should be flush with the end cap of the barrel of the tester -- which is the calibrated 12kg mark. In practice, some flesh of your thumb enters the barrel which can result in false positives. Again, its important that you practice with a scale before actually using your mask punch.
If the punch still rests on its conical section, the

MASK IS GOOD
else if the conical portion has completely entered into the mesh and the punch rests on its shoulder, the
MASK IS UNSATISFACTORY