Exercises 21 to 40 will focus on tactics that involve skewers in some way or another, ranging from easy to hard.
1… Qh1+ 2. Kf2 (or e2) Qh2+, followed by Qxb2
The two important tactical targets that black exploited here are the exposed king and the hanging piece (bishop) on b2. This again proves why it’s dangerous to place your piece on a square where it is undefended. It may seem safe for the moment, but it is a possible target.