This section is designed to prevent an abuse of group relief where, for example, because a parent company cannot, by reason of insufficiency of profits, benefit from capital allowances or losses of a subsidiary, it transfers the subsidiary to another group in a position to benefit by way of group relief from the subsidiary's losses etc. When the subsidiary's losses etc. have been stripped it reverts to the original group. The effect of the section is that where any arrangements are made by which a company may be detached from one group relationship and joined to another group or where it may be controlled from outside the group the company is to be treated as having terminated the group relationship. The same approach with certain necessary modifications is applied to consortia.
In other words, you simply just cannot transfer it out of the first group into the second group in order to transfer to the second group losses which cannot be absorbed in the first one, and then transfer it back again if it suits your book to do it.