To use a more concrete example (let's say that all months are 30 days).

Arrive Jan 1
first 20 days of Jan in Schengen
10 days out

20 days of Feb in Schengen
10 days out

30 days of March in Schengen

20 days of April in Schengen
10 days out

Now, you've used up your 90 days, and won't 'get' any more until July 2...and at that point it would be rolling...so the days spent in the zone in February will 'count' against your 90/180 day until august, and so on.

That is to say, you couldn't spend 90 days in April, May, and June, then leave and return the next day and stay July, August, and September.