Instead of following one another the sounds overlap; a sound which is acoustically perceived as coming after another one can be articulated simultaneously with the latter or even in part before it.
There's no subtext in Harry Potter really; it's all magic, anything can happen. Why do I say this? Because it's a magic spell. It's quite nice in a way. There is a real freedom to it.
When my friends and I would act out movies as kids, we'd play the guys' roles, since they had the most interesting things to do. Decades later, I can hardly believe my sons and daughter are seeing many of the same limited choices in current films.