Session breakout strategies mark the range of a quiet period, usually the Asian session, and trade a break of it when London or New York opens. The London open breakout is the best known. Variants use the first hour's range or a fixed window.
The claim is that the London open brings volume that resolves the overnight range in one direction, and that the direction of the break tends to persist.
We tested session breakouts across the standard windows and instruments on 17 years of data with real trading costs, and found no edge. The break of the range was followed by a reversal as often as by a continuation, and the wide spreads around the open ate a large share of what the successful breaks earned.
The reason is that the open is where the day's direction is contested, and the first move out of the range is as likely to be the losing side's attempt as the winning side's.
If you trade it anyway, measure the spread at the moment the rule fires, since that is when it is widest, and count false breaks honestly.