From the wikia:
From Latin everte, meaning "to throw out" and statum, meaning "stand". This implies that it would cause the victim to be thrown from their standing position, which most notably coincides with the effect seen in the films.
So must your opponent be standing still in order for this spell to work? That seems very impractical, even as a dueling spell.
It's never specified. Everte Statum is only used once, on a still opponent, then never again.
Everte Statum is only used once in all the movies, by Draco in the dueling club on Harry. It's never cast again or even referred to after that. At that time, Harry was staying still, however this doesn't necessarily prove that the spell only works on a stationary opponent.
Everte Statum is never in the books, and neither are any spells using the words "Everte" or "Statum" that could possibly give us a clue. The only two instances of "everte" appearing in the books are as part of the word "reverted".
There's no actual evidence for whether Everte Statum can be used on a moving opponent. Even the Latin etymology isn't much help there. "Everte" means "to throw" and "Statum" means to stand. However, "stand" could either mean "standing still" or simply "in an upright position".
