Delineating recursive function call in emulation code of worksheet SUM function

Currently working through the book Power Programming with VBA for Excel 2019. Frankly, up until a few chapters in, they are leaving a lot to be desired when it comes to explaining the more advanced statement concepts in their sample codes. But on to my point: Could you please explain each step of the recursive call statement MYSUM = MYSUM(MYSUM, n(m))

When MYSUM is initially recursively called, the initial MYSUM argument value is 0, since it was initialized as being =0 in the assignment statement at the beginning. But doesn't the same occur in each subsequent recursive call? So each call is just adding the next call's return value to zero. I do not grasp the compounding that is supposed to occur, mainly due to the initialization of MYSUM as zero during each call of the function.

Option Explicit

Function MYSUM(ParamArray args() As Variant) As Variant
' Emulates Excel's SUM function
  
' Variable declarations
  Dim i As Variant
  Dim TempRange As Range, cell As Range
  Dim ECode As String
  Dim m, n
  MYSUM = 0

' Process each argument
  For i = 0 To UBound(args)
'   Skip missing arguments
    If Not IsMissing(args(i)) Then
'     What type of argument is it?
      Select Case TypeName(args(i))
        Case "Range"
'         Create temp range to handle full row or column ranges
          Set TempRange = Intersect(args(i).Parent.UsedRange, args(i))
          For Each cell In TempRange
            If IsError(cell) Then
              MYSUM = cell ' return the error
              Exit Function
            End If
            If cell = True Or cell = False Then
              MYSUM = MYSUM + 0
            Else
              If IsNumeric(cell) Or IsDate(cell) Then _
                 MYSUM = MYSUM + cell
            End If
          Next cell
        Case "Variant()"
            n = args(i)
            For m = LBound(n) To UBound(n)
               MYSUM = MYSUM(MYSUM, n(m)) 'recursive call
            Next m
        Case "Null"  'ignore it
        Case "Error" 'return the error
          MYSUM = args(i)
          Exit Function
        Case "Boolean"
'         Check for literal TRUE and compensate
          If args(i) = "True" Then MYSUM = MYSUM + 1
        Case "Date"
          MYSUM = MYSUM + args(i)
        Case Else
          MYSUM = MYSUM + args(i)
      End Select
    End If
  Next i
End Function