Randomizing a List

I have this code, and it's working well. The input list is randomly shuffled very nicely.

But I'm having a little trouble understanding WHY it's working.

Specifically, what role does oBuffer play in all of this? Why the line at the end:

oBuffer(iRandom) = oBuffer(iIndex)

I find that when I comment it out, I end up with duplicates in the output list.

I've stepped through the code, but I'm still at a loss. What exactly is going on here?

  <Extension>
  Public Iterator Function Randomize(Of T)(Instance As IEnumerable(Of T), Rng As Random) As IEnumerable(Of T)
    Dim oBuffer As List(Of T)
    Dim iRandom As Integer
    Dim iIndex As Integer

    Instance.ThrowIfNothing(NameOf(Instance))
    Rng.ThrowIfNothing(NameOf(Rng))

    oBuffer = Instance.ToList

    For iIndex = 0 To oBuffer.Count - 1
      iRandom = Rng.Next(iIndex, oBuffer.Count)
      Yield oBuffer(iRandom)
      oBuffer(iRandom) = oBuffer(iIndex)
    Next
  End Function