Sorry for starting so many threads lately. I do that when I'm about to make a new board.
The explanation I heard that stuck in my mind is that the diamond shortens the rail line and makes the board feel shorter when on rail.
Any first hand experiences?
No first hand experiences. However Aruka's explanation of a diamond being similar to a rounded pin makes sense to me - if it is a reasonably deep diamond. But I don't think it would turn like a shorter board unless a compensating fin position adjustment was made. Either move whole cluster up a little or just move the rear thruster fin up, or a little bit of both. Not much adjustment or the board will be too loose.
My last 2 annual builds have been an elliptical tail - rounded pin without the actual pin. Last year's I overdid the compensation - was a bit too loose. Had all sorts of other things wrong with it too.
This years is perfect, whole cluster moved up just a tad, rear fin pushed a tiny bit closer to front. Position achieved by sliding Griffin twin tab in FCS II boxes in last years build with a bit of a guess on this years box positions. Losing last year's bump wing and reducing concave between fins to just a few millimetres also helped.