"Yes, Yuugi... in bed, too." Patchouli nodded. Honestly, sleeping with Yuugi wasn't so bad! It's just that she didn't like being interrupted when reading.

"However, I think we're going to need a little something to ensure that this challenge can't be tampered with. Since it's just us, though... could you please put your hands behind your back for me really quick?"

A simple restraint spell would help ensure that Yuugi wouldn't try to touch herself. Well, or anything else. That would mess with the results! Maybe that was kind of mean in its own right, assuming Yuugi would have that much difficulty holding back.