I see I have been mentioned in this thread so
I can take that as an opportunity to join this thread.
The trouble I, and others, have is with the term 'positional sacrifice'.
Let us look at the Oxford Companion to Chess
one of the most meticulously researched books on the subject of Chess.
In the section titled Positional Play
Hooper and Whyld state:
"The distinction between positional and tactical play is not clear cut.
All good moves have a strategic purpose and most contain tactical elements."
"...not clear cut." When does positional become tactical and visa versa.
In the section titled 'Positional Sacrifice"
"The distinction beween a positional sacrifice and any other kind
is somewhat fuzzy."
So here we have a book praised by everyone including Edward Winter,
meticulously researched and yet when it comes to explaining exactly what
is a positional sacrifice the best they have is...'fuzzy.'
The fuzziness (for me ) comes with how you judge a position.
Suppose White sacs the exchange to achieve a Knight outpost.
I prefer the term positional combination.
You have played a forced series of moves to achive an aim. (a Knight outpost).
Capablanca called them his 'wee combinations'.
In the case mentioned the Knight dominates the board.
The value of the Knight has risen to that above a Rook.
So Where is the sacrifice?
After a sacrifice you are meant to be down material.
In the case of sacrificial attack against the King one
can be two or even three pieces down and the remaining pieces
hoplessly out of play delivering mate in the corner of the board.
It matters not, the game is over.
After a correctly played so called positional sacrifice
the value of your remaining pieces or pieces has risen
beyond the scope of what they were before the sac was played.
So where is the sacrifice?
Spielmann covering the Positional Sacrifice in his book
The Art of Sacrifice
"However, if when all is said and done the positional sacrifice
is only an exchange..."
My reckoning is and everyone who has ever played a postional sacrifice
must agree. (else why play one) is that their position has improved
as a result of the sacrifice and their piece or pieces have much
more scope and mobility.
They have judged the position is better for them even though
they is no way to calculate it out to a win.
Where is the sacrifice?
A closer term would it's a postional exchange.
(stick 'The Positional Exchange' on the cover of a book and see
how many copies it sells.)
You also have the term Positional Sacrifice covering defensive moves.
You sac the exchange so your opponents pieces have lost their mobility
and the attack falters.
You are not improving your position, you are improving your chances
of not getting beat. 'The Positonal Back to Wall Sac.'
The only case of what I would call a genuine Positional Sacrifice
is what Spielmann calls an Obstructive Sacrifice.
He gives this example: White to play.
White played 4.e6. A genuine sacrifice, not a exchanging a Rook
for a Knight or even a Queen for a couple of minor pieces.
Black will have trouble getting his Bishop's out,
the Black King's position has been weakened.
This sacrifice with the majority of the White pieces still on the
back rank and no clear calculated win in sight is, if there is such a thing,
a positional sacrifice.
But the term Blockading sacrifce or Developing Sacrifice suits just as well.
It's played 4 moves into the game, you could claim it was a gambit pawn.
I think a strategic combination or positonal combination
is a better term than a postional sacrifice.
Not all combinations have to end in mate or the win of material,
in this case of an exchange sacrifice combination you have improved your
position and the tactical scope of your pieces.
A strategic exchange or positional exchange may even be better.
But sacrifice. No.
But as I said somewhere up there it all comes down to how you think
about the game.
I sacrifice pieces to get at the King. I don't want them back.
I don't want to see them again, I don't care about them.
The only weak squares I want are those around the King.
I will sacrifice every active piece I have to get at the King.
I have never seen a positional checkmate.