Fly up to somewhere approaching the top of the window and start a downward turn by pulling the left hand. As the kite turns left and the nose passes the 9 o'clock position, pop the right wing with a gentle axel-like motion. This causes the right wing to be pulled towards you and the nose of the kite to lift up so that it is spinning flat on its back in an anti-clockwise direction (looking from above).
If you think how a normal Axel forces the nose down into a flattened spin, this inverted Axel, the Backspin, forces the nose up into an inverted spin.
As the nose of the kite approaches the point directly away from you (the kite is still on it's back, but now has the trailing edge towards you), pull gently on the left line to spin the kite around on its back another time. Pop again at the same point to force another rotation, and so one.
Done correctly, the move should be very smooth.