Essentially, you have to bounce off the shell and hit the shell as Mario is still bouncing upwards. It's not a very easy trick to do multiple times in succession, but there's a few tips to help with it:
- Hit the shell as low as you possibly can, of course. You might have to manipulate Mario's position a bit if you're falling fast.
- Make use of the shell's bounce when it lands on the ground. Note that the shell must already be stopped before touching the ground to use that.
- When you land on the shell, try to hold B for as many frames as you can. I recommend waiting until Mario is closer to the height of the bounce before holding it down, as this will make the bounce longer than pressing it for a few frames at the start of the bounce.
There's not really much else you can do to help it other than that, though. As I said, it's a really difficult trick to get several times in succession, so it's probably going to take a while.
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