Trying to solve world 3: A Tingling, but can't access 8x time reverse

Solution 1:

Along with the commentors, I can confirm that tapping the Down arrow will move you back in time, and tapping the Up arrow will move you forward. (As long as you are holding down the Shift, you can switch between using fast-forward and fast-backward as often as you like -- when you let go of the Shift, fast-forward can no longer be used, since the game is already progressing forward at 1x).

My version of Braid also does what yours is doing: when I hold the Shift and press the Up arrow a few times, the screen will turn dark and it says "8x" but nothing happens. Just keep holding the Shift and press the Down arrow a few times, and you'll start moving in reverse.