This skill actually should be taught before a tuck because undoing a tuck is difficult. A tuck is just a layout with bent knees. To do a layout you must punch the floor out of a back handspring and stay tight, with core engaged, hips and glutes stretched and arms by the ears. As the body lifts with the toes leading the rotation, the body closes like a door on the arms at the upside down point or vertical to the floor.
You do not pull your arms down by your side. This has led to many, many early fulls. Extended prone holds and candlestick exercises will help your athlete understand the concept of the layout or hollow position. There is absolutely no bend in the middle of the body or arch through the shoulders in a layout. The arms lead the skill up into the air like the athlete is reaching up for a bar. If the arms are in front of the ears the layout is being led by the head and shoulders. The arms job is to lift your center of gravity upwards so they must do their job. This requires the round off back handspring to be performed technically correct with arms never leaving ears.