Head impulse - covert saccade
An abnormal impulse hidden inside the head thrust.
What the animation does not show. The exact timing that separates a covert from an overt saccade is an instrument reading, not a fixed bedside number. This clip stylises the timing to make the hidden saccade visible; do not read a millisecond cut-off from it.
The catch-up saccade lands while the head is still moving, masked in the blur of the thrust rather than clearly after it. It is easy to miss with the naked eye and is why the video head-impulse test was developed to time it. It is still an ABNORMAL, peripheral result - a 'normal-looking' impulse deserves a second look before you trust it.
A covert saccade is a real, peripheral catch-up saccade that fires while the head is still moving — hidden inside the thrust rather than landing cleanly after it. To the naked eye the impulse can look normal, which is the trap: a normal-looking bedside impulse in acute continuous vertigo deserves a second look, and video-head-impulse testing was developed precisely to time saccades the eye cannot catch (MacDougall & Curthoys).
Source: MacDougall & Curthoys (video-HIT); Halmagyi & Curthoys 1988.