In case anyone else is looking for a source a good search term is probably the Beal Effect. From the original paper by Beal and Smith:
Once the effect is pointed out, it does not take long to arrive at the conclusion that it arises from a natural correlation between a high branching factor in the game tree and having a winning move available. In other words, mobility (in the sense of having many moves available) is associated with better positions
In case anyone else is looking for a source a good search term is probably the Beal Effect. From the original paper by Beal and Smith: