The Xetra opening auction and the 9 am to 10:30 am window

9:00: the first price everybody has in their system

At 9 am, Xetra sets the opening price of each DAX stock: the price at which the largest volume of orders accumulated during the call phase can execute. The DAX opens at the value those 40 prices imply, and the FDAX aligns within seconds.

That price has a property almost no other level of the day has: a very large number of institutional participants have it in their system. It is the reference price of "at the open" orders, of algos that measure their performance from the open, of desks that note the gap with the previous close. You mark it, converted into the future, and you keep it all day.