- Time. When king and queen are knight forked, there is no time to safe both.
- Space. When you entomb a bishop, it has no squares to flea to.
- Function. When a piece has a duty, it has to stay in contact with its duty. Either it has to sacrifice itself, or it must give up its duty.
|White to move|
1r6/3qn1k1/R4bp1/3p3p/1p6/1N2P1P1/1P1Q1P2/5BK1 w - h6 0 1
Blacks Q, R, N and B are all more or less mobile. Only the king is immobile due to its function to protect the bishop.
Who is the most immobile piece now?
Q, R, N are all volatile still. But the king has now little mobility due to lack of space.
The king has four squares to go to.
2... Kg5 3. Qf4# lack of space = mate
2... Kf5 3. Qf4+ Ke6 4.Nd4# lack of space = mate
2... Ke6 3. Nc5+ lack of time = knight fork
2.... Kf7 3. Qf4 lack of time = double attack
Immobility in one form or another is the best guide for our attention.