The human link as a feat of strength comes in many shapes in sizes. The basics of it are to stop two opposing forces going in opposite directions from breaking your grip or hold.
Louis Cyr preparing to hold two horses back in the human link feat
Here you see the famous strongman Louis Cyr getting setup to stop the pull of two horses. Because of the way the feat is performed the human body can withstand great forces. And this is why horses or other strong things are used to pull.
A recent example is Dennis Rogers, who has held two planes from taking off. And on many occasions the pull of four Harley Davidson motorcycles. And below is a video of strongman Rene Golem Richter doing an interesting take on the human link feat.
Another human link feat can be done just using the grip of one hand. Objects are placed in the hand with ropes attached to them. And in this case usually two or more people on each side try to force the hand to open up.