Let's start with a simple position, the removal of the guard:
|Diagram 1 White to move|
In diagram 1, two of white's pieces attack a black piece (the green arrows). Two of blacks pieces perform a defensive task (the red arrows). Every task takes a tempo. But the tempi are evenly distributed. One move is one tempo for each colour.
1.Nxd6 Nxd6 is an example of two single purpose moves. The white knight executes the attack by capturing, the black knight executes the defense by capturing back. Both pieces disappeared from the board, along with the tasks they were involved in.
1.Bxf7 is a multi purpose move.
If white plays Bxf7, the distribution of tasks has changed. Not only the black knight has disappeared from the board, but also the defensive task it was performing has ceased to exist. Now black is in trouble, since he has to perform two tasks (save the black bishop, take back the white bishop), and he has only one tempo to so. Since there is no such multi purpose move, black looses a piece.
This might indicate a general rule of thumb: "try to harass the opponents piece that performs the most tasks, both offensive and defensive."
Task counting helps in this case to find the right move order. 1.Bxf7 is better than 1.Nxd6.
The value of the pieces can change this simple approach. Have a look at diagram 2.
|Diagram 2 White to move|
Now 1.Rxd6 Nxd6 is no longer equal due to the higher value of the rook.
All of this is very simple, of course. In a next post I will look if the rule of thumb we found still holds true, and whether counting the tasks still is useful in more complex positions.