The black stones have the shape of a trapezium.
This is the British sense of the word trapezium where two sides of a quadrilateral are parallel. The American word would be trapezoid. See also http://mathworld.wolfram.com/Trapezium.html
If the marked stone were at a, the trapezium would become a bamboo joint, if it were at b, it would become a table shape, and if it were at c it would become a tippy table. Usually, one sees the trapezium in amateur games as an attempt to connect and move faster. However, one rarely sees it in pro games and that because it is not such a good shape. Here are the weaknesses.
Secondly, in this case the trapezium can be cut.Try to spot the weakness and verify at cutting the trapezium.