AutoCAD .NET :: How Undo Marks Are Added To Undo Stack

I have a problem understanding how undo marks are added to the undo stack.In the code below a fan of lines are draw in a random location.If you run MTest a few times, the AutoCAD Undo command will remove the fans one at a time.

If you run MMulti to draw 10 fans, the AutoCAD Undo command will remove all the fans in a single step.I want to be able to remove the fans one at a time.I thought that  a  StartTransaction / Commit  pair would act as markers for the undo stack, but this isn't working.

Tested on AutoCAD 2010  VS 2008 
<CommandMethod("MMult")> _
  Sub testmult()
        Dim i As Integer
        For i = 1 To 10
            Call testdraw()
        Next
  End Sub
[code]....