This is one of those items that parents typically buy after they’ve worked themselves into a frenzy over SIDS and a baby’s sleep. I fell into that trap too and purchased this item after quite a bit of research even on the topic.
In practice, we never used it more than once or twice and it was quickly banished to the closet. A newborn does not move that much, especially if swaddled. And if swaddled, the positioner is pointless as the arms are wrapped. If you need to tilt the child a little to help with acid reflux or gas, some have suggested tucking a rolled-up thin blanket under one side although the best recommended sleeping position for infants is flat on their backs with no bedding whatsoever under them.
UPDATE: It looks like not using this thing was the right call. The US FDA recently published a warning regarding possible suffocation risk with sleep positioners and recommending that all consumer STOP using any sleep positioners following reports of 12 (!) known infant deaths involving such items. DOUBLE FAIL.