The Magnetic Handcuffs are a pair of restraining devices used by S.H.I.E.L.D. to immobilize their prisoners.
One of the agents tasked with apprehending Steve Rogers at the Triskelion carried a pair of Magnetic Handcuffs, in order to restrain Rogers, as he was labeled a S.H.I.E.L.D. fugitive by Alexander Pierce, due to his possible knowledge of the things Nick Fury learnt about the conspiracy within S.H.I.E.L.D.
The agent, who carried the handcuffs concealed as the handle of a briefcase, dropped the case and managed to place the handcuff around Rogers' right wrist, and his right hand adhered itself to the wall of the elevator. Brock Rumlow seized the opportunity to use his Taser Rod to attack Rogers, managing to hit him at his back, but Rogers was not knocked unconscious and defeated the other agent besides Rumlow that was attacking him.
Rogers deflected another of Rumlow's attacks, using the Taser Rod to knock another agent unconscious, and libertated his hand from the wall, knocking all the remaining agents besides Rumlow.
Rumlow, armed with Rollins' and his own Taser Rod, revealed that the attack was nothing personal. Rumlow was able to hit him with the rods a few times, but Rogers was able to withstand its effects, and defeated Rumlow throwing him to the ceiling and then back to the floor. As he was unconscious, Rogers answered Rumlow's final taunt saying that it appeared to be something personal, and retrieved his shield, which he used to break the handcuff around his wrist.