Forwards lingering on the wing is normally a product of the “2/4/2” system. Essentialtly, this involves 2 forwards remaining in the left 15 meter channel, 4 in the centre channel and 2 in the right 15 meter channel. The aim of this system is to force the opposition defence to fill the entire field, from touchline to touchline. If a defence decides to fill the field, and mark the likes of Wallace out wide, the hope is that there will be gaps in the centre of the field to punch through. If the defence close those gaps, the hope is that there will be an overlap for the likes of Wallace to utilise.
The All Blacks run this system very effectively, which is why you will frequently see their hookers and backrows remaining in the wide channels.