The following exercise is taken from chapter two of Dvoretsky's Analytical Manual.
This is what Dvoretsky had to say about this chapter:
From our search for brilliant combinations, we turn to the more prosaic, though no less important issue: calculating combinations accurately.
In his book Think Like a Grandmaster, Alexander Kotov introduced the concept of a "tree of variations", that being the collection of variations which needs to be analyzed. He also enumerated three kinds of "trees" - the "bare trunk", the "shrub", and "variational debris." In this and the chapters that follow, we will be looking at examples of each kind of tree.
The "bare trunk" is a long, forcing variation practically devoid of alternatives. (In point of fact, there are almost always alternatives; but if they are of little significance, we can honestly assign our calculations this kind of label.)
For less highly-skilled players, the main impediment is the need to accurately foresee each of the many positions that come up over the course of calculation. Over a lengthy calculation, they are prone to lose the threat, and find themselves unable to continue the variation.
But for trained chessplayers, too, there are difficulties - above all, psychological ones. The deeper one goes into the variation, the stronger grow the doubts: should I extend this line? Did I calculate everything corectly? Did I overlook something important?
You can increase your confidence in your calculating by moving down the line without haste, stopping at each step to check carefully whether or not there might be a strong alternative, either for yourself or your opponent.
The game position is taken from Spragget - Browne, New York, 1987. Your task is to evaluate 32 Bxa5.
Even with the question phrased this way, focusing on analyzing one concrete continuation, it is still necessary to think, if only for a little while, about the starting position: who is better, and what will happen if White plays a quiet move.
For if White stands better, then we will not be satisfied if, for example, the sharp variation that we must calculate ends in a draw. We would also be justified in cutting our analysis short if we see that it would lead to a situation that would be difficult to evaluate, and involve considerable risk.
On the other hand, these circumstances do not require us to just give up on the main continuation, if we assess the starting position in our opponent's favor.
Evaluate the consequences of the bishop sacrifice on g6.
Evaluate the consequence of sacrifice on g5!