Sometimes it skips to the next word before you are finished a word, and it's not clear if you need to press space or keep typing, but pressing space skips to the next word after THAT. Pretty frustrating, actually.
If you don't press space, you're violating the idea of words. If you press space, the sentence you've constructed in your head is broken. Either way, it slows me down and frustrates me too much to play :)
The original study used only a timer, and had the same frustration which you speak of here.