Basic Points

• The power hour is the last hour of the normal trading period, which is between 3pm and 4pm EST. It is not uncommon for very large moves to happen during this last hour, when otherwise most of the total move for the day roughly takes place in the first hour. 
• Generally, the first and last hour of the cash session has the most trading going on and also the biggest moves. This is usually when the most rebalancing (repositioning) is going on by major institutional players. 
• Since the middle of the day tends to be noisy in what is known as midday or "lunchtime chop", many traders will only focus on the periods near the open and close. 
• By waiting patiently until near the close, traders can respond to very large moves which might continue throughout the day.

Advanced: Getting to the Bottom of Rebalancing

With respect to rebalancing, delta is usually the only Greek which needs to be assessed because most of the major second-order Greeks (gamma, vanna, and charm) all translate directly to delta. This means that by hedging delta, most second-order Greeks are also hedged indirectly, including some third-order Greeks like speed (change in gamma vs change in the underlying security). At the end of the power hour, the major index options are still open until 4:15pm ET.