There are positions where a player amasses a whole herd of pieces at the enemy king's doorstop. In such cases it's obvious that a big attack is on the way. But sometimes there isn't any such buildup, and yet an attack is possible and may very well succeed. How is this possible?
One answer is that although the prospective attacker may have but a few pieces in the vicinity of the opponent's king, it might be that the defender has even fewer pieces to protect it. I believe that this explains how Topalov got crushed in game 4 against Anand from what looked like a fairly innocuous position, and it likewise helps explain the success of Larry Christiansen's brilliant attack against Yasser Seirawan in a 1978 contest.
The games are instructive, entertaining and beautiful, so have a look here. The show is free (free registration required) and will be available on demand for the next month or so.