Will Le
Velocity constantly change: 1-v (top) → 1 (right) → 1+v (bottom) → 1 (left) → 1-v (top). That means it constantly accelerates from 1-v (top) to 1+v (bottom) at acceleration 4v then constantly decelerates on the way back. When parameter v varies in the range [-1, 1], the velocity at the horizon (left up and right down) is kept unchanged (= 1), and the corresponding acceleration a = 4v varies in the range [-4, 4].