The definition of a trapped surface in Sean Carroll's "Spacetime and Geometry" is as follows.
"A compact spacelike, two dimensional submanifold with the property that outgoing future directed light rays converge in both directions everywhere on the submanifold."
The light rays emanating from two spheres inside an event horizons would evolve to smalled values of radius for both outgoing and ingoing rays. I don't understand the intuition behind this explanation. If something is moving outwards how do they converge in the future and merge into singularity.