Quick attack Cancel is just Quick attacking into the ground, Simple as that.
And it doesnt have to be directly down on the ground either.
From that point you can:
-Go Directly into an attack ( Thunder, Another Quick attack, or dair )
-Jump, Than attack ( Uair and Nair are the most commonly used, but you can also do another QA, Thunder, fair, dair, or bair. )
-Land, And do a ground attack rather quickly ( usually a tilt, or shield )
To keep low to the ground you can ( asuming you are on the left and samus is on the right ) Quick attack East, 2nd Southeast, So its basicly two Quick attacks along the ground. Its a QAC because the second is directed toward the ground, from there you can go into any of the attacks that can be used out of QAC.
You could also just QA Southeast, then attack. if you dont have to move as far.