The problem with these practice sites is that it's a series of still images, so each letter is distinct. Real-life fingerspelling is vastly different because people don't always form their letters clearly, and even those who do, the movement from one letter to next can sometimes obscure letters.
Still images also change the "configuration" of the word, that is the shape of the letters and the movement of the hand as it forms them. Some words have such a distinct configuration that you can recognize them without actually seeing the letters, but you don't get that effect with a series of still images.
This site could also train you to focus on the hand which is a bad habit because you're so rarely looking at the hands while communicating with someone in sign (and if you are, stop it! Be polite and look them in the eye!). If you're not trained to catch fingerspelled words in your peripheral vision then you'll have to change your point of focus, by which time you will have missed the first few letters -- or even the whole word if it's short -- and then the person goes right back to signing, and you're lost.
In short, I'm not convinced these fingerspelling practice sites have much value.
