Tendex and vortex lines describe the gravitational forces caused by warped space-time. They are analogous to the electric and magnetic field lines that describe electric and magnetic forces.
Tendex lines describe the stretching force that warped space-time exerts on everything it encounters. “Tendex lines sticking out of the moon raise the tides on the earth’s oceans,” says David Nichols, the Caltech graduate student who coined the term “tendex.” The stretching force of these lines would rip apart an astronaut who falls into a black hole.
Vortex lines, on the other hand, describe the twisting of space. If an astronaut’s body is aligned with a vortex line, she gets wrung like a wet towel.