DSum(Expr As String, Domain As String, [criteria])
You can put anything in that first argument. Though it is mostly used to return a fieldname it doesn't have to be. It can be useful for returning multiple fields formatted to your heart's content. For example, using a DLookup for a full name from multiple fields in a single command ControlSource.
Meanwhile any expression in the context of the form is tried against the controls and fields collections of that form. So when the literal was fed to the DSum instead of a string, the value of the control by the same name was read.
The DSum dutifully summed the integer fed to its Expr argument, just as if you had written:
SELECT SUM(2345) AS A FROM whatever
You get the number of records times the integer.