grasp (someone or something) by (something)
To take hold of someone or something by gripping something in particular. The mama cat grasped her kitten by the scruff of its neck and carried it off. I grasped Peggy by the shoulders and guided her down the hallway.
grasp someone or something by something
to hold onto someone or something by something. He grasped his friend by the hand and pulled him to safety. Sharon grasped the dog by its collar and held on tight.