Successive frames show downward continuation to successive depths where best focus occurs.
The waves focus best at zero offset. The focus represents a downward-continued experiment, in which the downward continuation has gone just to a reflector. The reflection is strongest at zero travel time for a coincident source-receiver pair just above the reflector. Roughly it amounts to the same thing as the conventional procedure of summation along a hyperbolic trajectory on the original data. Naturally the summation can be expected to be best when the velocity used for downward continuation comes closest to the velocity of the earth.