The Point Between Life and Death is the 216th chapter in Tsubasa: Reservoir Chronicle.
Yuko, still in Clow Country, reveals that she was protecting the world where Sakura Li and Syaoran Li currently live in. This world is actually the tube/hourglass seen in the beginning of the series. This "world" was entrusted to her by Toya and Yukito. By their own powers, they turned back time themselves and left their "children" (Syaoran and Watanuki) behind. The price they paid was their ability to touch each other, unable to do anything but just watch. As Yuko's time moves on, and like Sakura she's been kept from death, she'll use a dream to pass the powers of Sakura Li and Syaoran Li. Yuko breaks the tube, releasing them.