Chess Principles

I just received the book 800 Chess Traps by Bill Wall. I have not even looked at the traps yet. In the pre-fix of the book he has a list of Chess Principles. I thought that was pretty good and circled ones I needed work on, like "don't take unnecessary chances". I see some great possibilities and will sometimes go for it without see far enough to really know the results. My questions are for

15. When behind, exchange pawns. When ahead exchange pieces.

I understand the pieces part, but if behind why not keep the pawns for a possible promotion later in the game? Also, the pawns are so important in position, it seems too general.

35. Trade pawn pieces when a head in material, or when under attack. 

The first part kind of makes sense, but contradicts principle 15 somewhat. what about the second part?

Any feedback would be appreciated. 

"When behind, exchange pawns. "

You exchange pawns when losing to reduce the number of pawns that can promote.

Yeah, this is why I don't follow guidelines too dogmatically. They are useful to keep in mind (like chess opening principles), but best to understand why they work when they do, so that you'll be able to better judge instances where there are exceptions.

"When behind, exchange pawns. When ahead exchange pieces." The pieces part is easier to understand and the part worth remembering; the pawn part of it only works in some cases. The logic behind exchanging pawns when down in material is to force an endgame with only pieces (no chance of pawn promotions) when a draw might be held. A similar reason is because more pawns gives the player up in material, more resources to work with.

With no pawns available, the endgame might be drawn like this:

In both endgames posted, it is a "simple" draw regardless of how the pieces shuffle. As long as the defender (despite being 2 points down in this case) keeps the minor piece close to their King (and preferably King near the center of the board), then they should hold the draw easily.

"Trade pawn pieces when a head in material, or when under attack." You want to trade pieces when under attack because you'll be exchanging off potential attackers (especially active ones already). As a guideline, an attack in chess requires at least 2 more attackers than defenders to successfully checkmate or gain a decisive advantage somehow. 

A younger Kasparov references this attackers versus defenders concept around 4:48 into the video.

p.s. I had two diagrams illustrating a draw, but they didn't display  

One was R vs N and second was R vs B; both near the center of the board and it is an easy draw.

"When behind, exchange pawns."

in some situations where you are behind material, if you can exchange enough pawns, the endgame might be a draw. this is common in rook vs minor piece endgames (lone rook vs lone bishop/knight is usually a draw), and minor piece endgames (knight/bishop vs lone king is a draw, knight+bishop vs lone bishop/knight is a draw, etc.)

"Trade pawns and pieces when...under attack."

if you're able to trade off some of your opponent's pieces and pawns that are fiercely attacking your position, then slowly your opponent's attack will die out. 

but of course there are always exceptions to both of these principles

From the book Applying Logic in Chess by Eric Kislik

In the section talking about various advice given to beginners that isn't always entirely true

"When you are ahead, exchange pieces, not pawns"

While there are not many situations in which exchanging pawns in profitable for the side that is material up, in some cases, trading pawns does make converting a win easier for a practical player at the board. This can happen when certain pawns and hard or awkward to defend, and trading them off simply gives you less to worry about. A lot of people get confused by the idea of exchanging pieces while material up and overfocus on it, instead of focusing on playing good moves.

"15. When behind, exchange pawns. When ahead exchange pieces."
Pawn endings with an extra pawn are usually won. Rook endings with an extra pawn are often still draw. The less pawns, the more likely a draw. KRPPPPP vs KRPPPP is won, but KRPPPP v. KRPPP is already drawish. Even KRfg vs. KR is a draw. If you are a pawn behind trade pawns and do not trade pieces to increase drawing chances.

"35. Trade pawn pieces when a head in material, or when under attack."
When you are a pawn ahead, trade pieces, not pawns to win the endgame.
When you are under attack, trade pieces to ease the defence of your king.

I am pretty sure that the opening poster misquoted 35. The way it is written here is pure nonsense. I suspect at least 3 typos in that sentence.

Ok, I will fix all this in my next book.  Thanks.  - Bill Wall

Guidelines are great if you understand them (so you can have a grasp of their limitations too).
"When behind, exchange pawns. When ahead exchange pieces."

The idea is that the winning side wants to reach the endgame as fast as possible, having an extra pawn with no pieces other than pawns and king on the board is an almost won position.
As the losing side you want to get rid of pawns the winning side needs to promote and keep the action as complicated as possible. lone kings and bishop+knight vs single bishop is a draw while 1 pawn vs two pawns is generally winning for the side with two pawns.