A single zoomed snap was placed over a static exposure, and a gradient mask was used to reveal only the lower portion of the zoomed layer. That same layer was then reduced in opacity to let the foreground buidings have more definition (rather than blurred).
It took quite a few tries to get this particular effect. Key, as I found out giving it another go on the evening of 9/11, was the placement of the center focus point. Fine adjustments seem to matter, to get the streaks aligned with the perspective. When it was off, it was very uncomfortable to have the streaking not aligned with the row of windows about them. 2/3 determination, 1/3 serendipity.
Each attempt was two shots (zoomed & static) with wide variations start/end points, speed of zoom, static shot FOV, blah, blah. Out of 250+ exposures over 2 nights, I was only pleased with 2 sets. Mind you, that wasn't all about zoom effect - many were about the changing atmosphere about the lights.
