AT40 played it last sunday, but didn't play it this sunday. Because AT40 has a huge effect on the amount of spins, it was played less this sunday than last sunday, so the update is negative.
That's the rolling effect.
A general explanation is more difficult, and Arjun already tried, but I'll give it a shot
Basically it just means that: spin update = <amount of spins today> - <amount of spins 7 days ago>.
So the spin update of -28 means it had 28 less spins this sunday than last week sunday.
The rolling effect means that if last week had a high amount of spins, then this week will have to compare to that high amount, making it more difficult to get a good update.
That's the best I can do to explain it
Wait I actually got it, watch me forget about it on the next single though
