A Cactuar dash is a dash back out of a Run cancel. Characters like Marth and Sheik benefit the most from this, since it compensates for their mediocre dash dance. Fox, on the other hand, is already blessed with divine dash speed, run speed, and dash dance length, rendering this technique quite situational at best.
On Controllers
It is well known that some GameCube Controllers can't dash back as well as others. Luckily, we have Universal Controller Fix (UCF) to make controller luck less of a factor in competition. However, dashing back out-of-crouch differs from standing dashback is still inconsistent for two reasons.
First, there is a small coordinate range which, if polled, will cause you to stand up without a dash, leaving you with a buffer walk instead. This, by itself, adds an element of luck to doing it consistently. The late Hax$, peace be upon him, called this 'Dashback out-of-crouch Y' or 'DBOOC Y'
More importantly, though, dashback out-of-crouch is a two-frame window. Good, right? For standing dashback, that's what players are used to. But moving the analog stick from the centre to the side is faster than from the bottom to the side, whether you quarter-circle or go in a straight line. The square root of two is greater than one, after all.
For those reasons, Cactuar dashing is unfortunately burdened with an element of polling RNG; the same polling RNG that can make tech-chasing from a crouch inconsistent.