From my understanding, and I’m sure someone more knowledgeable than me will corect me if I’m wrong
From the point you start work, the 24 hour clock is ticking. Before that 24 hours is up, you have to complete 11 hours of rest. If you work for 13 hours and one minute, you cannot complete 11 hours rest before the 24 hours is up, therefore you’re in a reduced rest. This you can only do three times a week (midnight sunday/monday to midnight sunday/monday).
However, like Franglais said, you can split your daily rests by having a 3 hour uninterrupted break during your shift and a 9 hour rest at the end. You can do this as often as you like. Why youd want to though, I dont know.