question about "freeze" with PFFT~

Cheng Chien-Wen's icon

I was wondering if anybody knows how this subpatch works.
It is a subpatch in pfft used to freeze a frame.

I would like to know why the spectral frame size and bin index are added together rather than multiplied by each other. Could anybody explain a little bit ?

Thank you very much.

Chien-Wen

Max Patch
Copy patch and select New From Clipboard in Max.