The Loop has the ability to make a specific area 'repeat' itself, so to speak. when a person or being tries to walk out of the area that Loop is affecting, they are sent back to where they entered the afore mentioned area, which will never change. The Loop's main body is a simple, red and black Möbius strip, but appears in pairs, one at where the 'loop' begins and one where it stops. It is under The Earthy.
The Loop first appeared in Episode 21. Loop had affected a part of the trackfield at Sakura's school during a school race. Sakura and Syaoran start running at top speed when they see that Yukito is watching. But The Loop makes them restart at the beginning of the track when they get close to the finish line. After they realize what happened, Sakura and Syaoran find the 'beginning' and the 'end' of the area Loop was affecting and find two identical main bodies of Loop, one at each location. Sakura and Syaoran tried to cut the main bodies one at a time, but they would just repair itself. Then Sakura and Syaoran tried cutting both main bodies at the same time with The Sword card and Syaoran's sword, respectively. After both main bodies were cut, Loop's ability faded and Sakura was able to seal it and become its master.
The Loop was transformed into a Sakura Card in Episode 53. After Sakura fainted in the middle of her frantic rush of transformations, The Dash fled due to being stuck between Clow Reed's magic and Sakura's and possessed Touya's bike and sped around town. Sakura used Loop to keep Dash in one area as she tried to catch up to it so she could calm it down and fully transform it into a Sakura card.