A positive BFS proves to the market that the project in question...

A positive BFS proves to the market that the project in question is viable (or not), and depending on the economics, could significantly move the share price. Further movement in the share price would be expected as various development and approval milestones are reached.

The first letter in BFS stands for "Bankable". In other words, the study is sufficiently detailed and robust for a financier to decide if the project is worth funding.