I think you have it. The only trick is that it schedules the timers in 500 msec increments. That is what the i*500 does. So the first item in the list of open rollershutters will have a timer run immediately, the second one 500 msec after that, the next one 1000 msec after the first one and so on.
See Design Pattern: Working with Groups in Rules for more stuff you can do with Groups.