The disconcerting hole in the top of the parachute canopy is called an apex vent.
Before the apex vent, the only way that the air trapped underneath the parachute could escape was to spill out from one edge of the canopy, thereby tilting it and throwing the hapless parachutist to one side.
As the canopy swung back, more air would spill out from the opposite side, setting up a regular, pendulum-like oscillation.
Watch any footage of Second World War parachutists and you will see this.
As you can imagine, hitting the ground during a downswing was understandably hazardous, especially if it was also a windy day.
The apex vent, by allowing the air to leak slowly out of the top of the parachute canopy, prevents this wild oscillation and makes for much safer landings.
Another benefit of the apex vent is that it slows down the opening of the parachute.
Without the vent, air inflates the canopy much more abruptly, and it can damage the parachute or bring tears to the eyes of, particularly male, jumpers.