I know exactly what combo you're talking about. It's #7 on Sly's combo tutorial. Here's a video of it:
It's not impossible, as you can see it being done in the video. But, I tried doing it for nearly 20 minutes during the tutorial and still couldn't do it. So, I figured if I couldn't do it during the tutorial I'd never be able to do it in battle and I just skipped over it. There are other combos like that, such as Good Cole's last one. I suppose only black belts use these techniques.
Btw, the video makes it look easier than it is.