I guess you were reading about aspiration search.

When you call alphabeta with a window (alpha,beta), the returned value will be alpha or less if the minimax value of the tree is alpha or less, it will be beta or more if the minimax value of the tree is beta or more, and it will be the exact minimax value if it is more than alpha and less than beta.

In code:

v = alphabeta(position, alpha, beta, depth);if(v<=alpha || v>=beta){  // the window was too small}

[Edited by - alvaro on July 9, 2006 2:51:23 PM]