All pulse oximeters, those working in hospitals or consumer version, have difficulty working if the sensors move in relation to the skin, preventing the pulse oximeter from achieving a good reading.
This is due to the nature of pulse oximetry, and how the technology works.
As depicted below with a standard finger-clip pulse oximeter, two light sources shine through the skin and capillaries. Some light is absorbed by the blood and the remaining light is then measured again. The difference in light is used to determine through mathematics how much oxygen is in the blood as well as the heart rate.
(Two light sources, Red light and Infrared light shine through the finger.)
When the sensors move in relation to the skin, the light takes a different path through the tissue, and some light is scattered and lost, or absorbed differently. This makes the end readings inaccurate.
(If the finger moves, the light doesn't make a straight path, and creates a false read.)
To improve the situation:
**When the sock is worn correctly, with the toes all the way to the end and the bottom strap snug around the foot, it is less likely that the foot will move separately from the sock. Even with some kicking, the Owlet Baby Monitor will get better reads due to a good sock fit.