An extension of five lines on the side is easy to invade, perhaps. But Yang Yilun teaches that it is good as framework (ie with building territory in mind).
If Black is able to add , this becomes a good 'tent' formation.
Velobici: According to Yang Yilun, as I have understood my son's teaching games: A five-space extension from a corner on an open side of the board is good...it allows the either the building of territory or the creation of two strong positions on the side. A five-space extension is not good if it approaches a strong position of the other player.
An open side is a side of the board in which only corner plays have been made resulting in between 9 and 11 lines between the the two corner positions.