The short answer is that neither the Secretary of State nor a local authority has direct power to force a maintained school to become an academy.
They are doing it in practice either by (a) replacing the school’s governing body with an Interim Executive Board comprising people he has chosen who then act on their behalf in ‘applying’ to become an academy; or (b) by pressuring the existing governing body into applying themselves (i.e. jumping before they are pushed).
But neither can directly force a maintained school to become an academy.
In my experience, when governing bodies are approached (generally by private consultants employed by the Department rather than by civil servants) and told that the Secretary of State wants to turn their school into an academy, but they are not given any details about the legal processes involved. There is a strong sense of “jump or you’ll be pushed” which encourages many to jump.
Where they don’t jump, the local authority or Secretary of State imposes an Interim Executive Board which then acts on their behalf in going through the motions of a consultation etc with a view to initiating the academy conversion process as if they were a normal governing body (except that, instead of the process being initiated by a governing body which comprises parents, staff, community governors, etc, it is then initiated by a body of people put in place to bring about the forced change to being an academy).