motionBlur() smears the whole picture along one direction, as if it moved while the shutter was open. velocity is the length and direction of the smear in game pixels, as [x, y], and samples how many reads make it up (at most 32).
A game usually sets velocity from how fast the camera moved this frame, so the blur comes and goes with the speed. Here it is set by hand.
← → to turn it, ↑ ↓ for its length and Space to switch it off.