She can’t do anything else

Kara’s eyes narrowed as Lord spoke and her mind began racing. She absolutely did not like the sound of that, and she despised the way he stood behind her, as well as the feel of his breath on the back of her neck.

When he told her to put the cuffs on her wrists she did as she was asked, all the while keeping them behind her back like she was asked.

‘Wait what?’ She thought, suddenly confused. Why did she do that???

Suddenly the Girl of Steel was very confused. “What did you just do??” she asked, suddenly wanting to remove the handcuffs but something mentally is blocking her from doing so.

Lord grinned broadly, his pearly white teeth painting a perfect target for a punch, but safe in the knowledge that Supergirl no longer had full control. "Well, very interesting. Simple handcuffs of course. Normal metal. Easy to break. You could split them in a second with your Kryptonian might." He paused and took a step forward, raising a hand and taking Supergirl's chin in his grasp. "And yet you won't... or can't..." He smiled and cocked his head to one side, as if admiring a particularly fine piece of art he'd just purchased.

Kara was suddenly filled with dread at the sudden realization that she was completely and utterly trapped. She couldn’t get herself to move her hands out from behind her back, no matter how much she tried to mentally tell herself to do so, let alone break the handcuffs, and she couldn’t leave because she was told to see the lab. She was stuck, and she was at Lord’s mercy.