Using a bolster chisel isn't too difficult as long as you remember to ensure that the entire length of the blade is flat on the work. Bolster in overall terms means, to support. There are many different types of bolster support including bolster pillows but in the masonry sense the bolster chisel supports the mason by being used to crack open rocks before the work can begin on carving and cutting them into the required shape.
The thick section of a knife blade, just as it joins the handle, is also called the bolster.
Bolster chisels, with their relatively rounded edges, are designed to crack rather than to cut. The wide blade allows them to induce a crack into the widest of surfaces, such as masonry blocks etc. It also allows them to be used for cleaning surfaces of mortar, concrete and other spillages.
Its also a very good idea (unlike the young lady in the picture) to wear eye protection!
Bolsters can be bought with hand guards as shown in the image above and in general practice are used with a lump hammer for maximum effect in cracking stone, blocks or bricks.