Idk what its called but you can get it on Temu lol Look this up Temu 12pcs/set Resistance Pull Ropes, Boxing Explosion Strength Trainer, For Taekwondo, Indoor Fitness, Leg & Arm Training
Its pretty much resistance band training and its good for shadowboxing because it can force u to work harder during your trainings by making it harder to fully extend on your punch, so when u take it off you are faster and sharper. Most important thing is to find the right resistance in the bands (not too much not too little) and to make sure it fits your size and is made with good material