How does the inferring work on this line?

[coodoo]

TypeDD-Samples/Chapter6/Printf.idr

Line 27 in a5c08a1

printf fmt = printfFmt _ ""

Use an underscore (_) for the format, because Idris can infer from the type that it must be toFormat (unpack fmt).

This's the explanation from the book for above line but I'm still a bit puzzled by how does the inferring work here? Since fmt comes in as a String type, how does it become a Format type when passed to printfFmt?