These blades allow cuts to be made in the centre of a panel without the need to drill a hole first. Amongst other things this means that the section which has been cut out will fit back into the hole from which it was cut, with only a small gap all round. An example of a use for this is making doors for dolls' houses and other models.
The most commonly used piercing blades are the H-150 Panel Piercing seen below,
and its big brother the H-240 Panel Piercing blade.
Both of the above blades are exceptionally thin at 0.3mm (kerf slightly more). Z-Saw also make 4 other saws with piercing capability and more robust blades, as follows:
H-180 Waistern woodworking and H-180 Waistern Panel blades
The 2 latest additions to the Z-Saw catalogue, the wonderful VIII265 folding carpentry saw and the V Handy Folding Carpentry saw shown below, can also start a cut in the centre of a board.
These 4 blades will fit any Group D handle (such as those for the H-250 and H-265 range of blades, as well as those shown in the pictures).