Of course there is also a yaw axis or better said yaw axis stabilization on the Spark. The yaw axis is just not stabilized mechanically like the roll and pitch axis. Yaw is digitally stabilized. This is done by using a 1920x1080 pixel box (crop) out of the 3968x2976 sensor. The box is dynamically positioned to counteract movement of the drone in the yaw axis. What you observe is the positioning of said box to the center of the sensor as part of the initial calibration after power on. If you have a close look at your footage you will notice a slight rebound after a harder yaw input. Same effect. It digitally repositions the 1920x1080 box to regain some wiggling room again for stabilization.