Who even said anything about teleporting the whole body?
The portal is moving into him so as the parts of him go through they appear on the other side like a door
Think of it as a portal falling on you and another on the floor next to you, as it falls your head goes through and your head appears through the portal on the ground and the rest of the body goes through as it falls until the portal lands on the floor and you’re standing on that same floor but in the other place as the portals are still connected.