Subsection (1) provides that, despite the transfer by virtue of section 53 of any function under certain specified enactments, the function will be exercisable by a Minister of the Crown as well as by the Scottish Ministers.
The specified enactments are:
Subsection (2) enables subordinate legislation to provide for any other function which is transferred by virtue of section 53 to be exercisable (so far as the subordinate legislation provides) by a Minister of the Crown as well as by the Scottish Ministers.  This power has been exercised in the Scotland Act 1998 (Concurrent Functions) Order 1999 (S.I. 1999/1592).
Subsection (3) provides that the power in subsection (2) may not be exercised so that subordinate legislation made under it comes into force at any time after the function in question has been transferred. That date was 1 July 1999 for the initial suite of functions transferred to the Scottish Ministers.  However, the power may also be used if, at some time in the future, further functions are transferred to the Scottish Ministers, such as in consequence of extending the legislative competence of the Scottish Parliament and therefore also the concept of devolved competence.
Subsection (4) provides that any power referred to in section 53(2)(a) (those of Her Majesty’s prerogative and other executive functions which are exercisable on Her behalf by a Minister of the Crown) to establish, maintain, or abolish a body, office, or office-holder having functions which include both:
shall, despite section 53, be exercisable jointly by the Minister of the Crown and the Scottish Ministers.  This enables the Scottish Ministers and the UK Government to establish jointly, for example, an advisory committee to advise both administrations about a matter of common interest (e.g. some environmental matter) or a company or trust to operate on a cross-border basis.
Subsection (5) provides a definition of “office-holder” for the purposes of subsection (4).
Commentary Part II: the Scottish Administration Details of Provisions