When the head and shoulders pattern happens during an uptrend, the price rises and then pulls back (lower), forming the left shoulder. The price rises once more, forming a higher peak known as the peak of the head. The price falls once more, then rebounds to a lower peak, establishing the right shoulder.
Swing lows occur when the price falls after the left shoulder and the head. A "neckline" is formed by connecting the swing lows with a trendline that extends off to the right. When the price falls below the neckline, the pattern is complete, and the price is likely to continue falling.
The entry opportunity on a head and shoulders pattern occurs when the price breaks the neckline.