Here White intends P-Q4. That point is dominated by Queen, Knight on KB3 and Pawn on QB3, it is defended by Bishop on QB4, Knight on QB3, Pawn on K4. The units being equal in number, White is safe in advancing the Pawn.
Though the rule as above stated holds unexceptionally good, it needs a corollary in that the "value" of the pieces engaged in the contest have to be taken into account. Generally it would be foolish to post a Queen, however well defended, upon an empty spot assailed by a hostile Pawn. To place a very valuable piece upon a contested point cannot be my intention, unless I obtain thereby a very valuable compensation.
The question becomes very much more complicated as soon as several points are under contention simultaneously. To obtain the superiority upon the spot A, I may attack one of the defenders of that spot situated on B, and even though my opponent have superiority on B, that attack may serve me to gain superiority on A.