Feet on the pegs is almost always better than feet off the pegs. However, I think the point being made is the fact that momentum is crucial in that situation (as it is in many situations). Sometimes paddling with your feet can help remove some weight from the bike allowing it to bounce over the rocks more easily keeping your momentum up. If that doesn't work for you, It's hard to go wrong with feet on the pegs standing.
...as if I knew what I was talking about.