birth control patch - Xulane?

So I was reading the directions on my prescription. It says to apply the patch during your period at some time and give it seven days for it to be effective... But it also says if you apply it the first 24 hours of your period, you're protected and no other protection is needed. I found this to be kinda odd. I mean, I believe it obviously because it's in the instructions but does anyone understand how that works? Lol