The direction of the jump depends on the duration of the motion inducer. For durations below 80 ms or so (the threshold varies from one observer to the next), the jump is usually seen in the same direction as the inducer (which can be changed using the radio buttons on the top right of the page). For durations above the threshold, the jump is usually seen in the direction opposite to the inducer.
Note that for very brief durations, the inducing motion is not perceived directly as it is masked by the transient; only its direction 'shines though', or is amplified by, the transient mask. The transient can be turned on and off using the radio buttons near the top.
Note also that for duration 0 (no inducer), the direction of the jump is undetermined by the stimulus. Some observers nevertheless perceive jumps—in arbitrary directions, that can sometimes be determined by voluntary decision.