This is another pretty straight forward idea -- your piece is being threatened and you find other pieces that can move to defend it. This usually happens when the piece is loose (or just underdefended) you don't want to or can't move the piece for some reason. The search method is simply to look for other pieces/pawns that can move to defend the piece. Add those specific moves to your candidate list to calculate further.
The idea of capturing/defending brings up another important element of piece safety -- counting. In its simplest form, the "count" is the number of attackers vs the number of defenders. However, in addition to the simple quantity of attackers/defenders, there are also issues of quality, which may trump quantity, order of recaptures, and also captures on several squares. This is a topic for another entire blog series, but it is very important to know the basics anytime you are considering defending a piece that is being attacked, and is hugely important when considering counterattacks (which we'll look at later).
Let's start out with a simple position to demonstrate the idea. The general process is to evaluate the position, identify and prioritize all threats, and then use the five defensive ideas to identify specific candidate moves. Next, calculate each candidate move to the end and evaluate the final positions (material and activity). As you progress through the candidates, keep in mind the "king of the hill" (KOTH) candidate move to help you pick the best at the end.
Black just played Qf5 attacking White's rook. White to move.
|2kr4/1pp2p2/p4p2/5q2/4RP1Q/P1P3PR/1P6/1K6 w - - 0 1|
Let's evaluate the position and then find all of white's threats. White is up an entire rook, and black has no clear compensation. White is playing to win, while black is fighting for a draw. Black's immediate threat is to capture white's rook with the capture-check Qxe4+, restoring the material balance, and black will be slightly better because of his more actively-placed pieces. Black has no other threats. If white can keep his rook, he is clearly winning. Let's look at the five defensive ideas to see what candidate moves we find, and then pick the best move.
Idea 1 (capture, pin, deflect): no captures; 1. Qg4 pins the queen to something more valuable, but it is unsafe.
Idea 2 (block): none.
Idea 3 (move): none -- the rook is pinned to the king and cannot legally move.
Idea 4 (defend): searching the board it is not immediately clear if white has any mvoes that defend the rook. It is tricky to find, but it is there! Highlight to see the answer: [1. Qh7.]
Idea 5 (counterattack with an equal or greater threat to WIN, DRAW, DEFEND): an equal threat would be to win black's rook for nothing, but that is not possible here. A greater threat would be to attack the queen or the king, but there are no checks and we've already considered 1. Qg4.
The tricky part of this position is not the calculation, so I'll skip further analysis since white is now clearly winning. If you couldn't find the move (but you did look), try training positions using the x-ray theme until you never miss them again. This idea is very important to remember -- if you find that you are "missing" moves, try to really understand if you missed them because you didn't even look, or you looked but didn't see. Those are very different problems to have, but both can be trained.
This is the last blog covering the basic four ideas. The next blog series will cover the very exciting and complex fifth idea of counterattacking!