Their models showed that lithospheric buoyancy forces were responsible for the more predictable, rift-perpendicular deformation, but those forces couldn't account for the anomalous, rift-parallel ...

If the buoyant force of a fluid is greater than the weight of an object placed within it, the object will float. Helium, being lighter than the other elements in our atmosphere, rises.

If these two forces are equal, then the object will be neutrally buoyant and neither rise nor sink. It's like hovering, but in water, and it is essentially what you want to do when scuba diving.