In a recent post from SpaceX about [Crew Dragon parachute test](https://twitter.com/SpaceX/status/1011703684454014976) you can see how the main parachutes are deployed in semi-folded state, after a while they inflate a bit and after another while they finally fully extend.

Images of the sequence steps captured from the linked video:

[![enter image description here](https://i.stack.imgur.com/BBg86.jpg)](https://i.stack.imgur.com/BBg86.jpg)

How is this sequence timed and driven? Is it just aerodynamic forces, are there any control strings or is it something different?