Had one like this last week.
Tackled close to the line ending up on his knees.
He then fell forward onto his front, thereby gaining another 3 feet or so.
He then reached out solely with his arm to place the ball on the goal line.
Opposition claimed it was a double movement, as they saw the fall forward as a second motion.
I conferred with my AR and he (like me) saw the fall forward as part of the tackle, so not a second movement.
The gap between being on his knees and falling forward was less than a second.
Thoughts?