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How to explain quick price changes early in the morning
The gap up/down and rapid movement immediately following market open is due to overnight futures activity. In your example, SP500 on June 20, 2016 saw a 20-point gap up at market open. This was because the SP500 futures were trading 20 points higher at 9:30 AM than at its close on Friday. The index will always "catch up" with futures at market open. You can see that below. The top chart is the E-mini SP500 futures from Sunday night to Monday. Beneath it is the SP500 index.