To be clear, you are talking about After Effects CS5, yes? My understanding (limited as it might be) is that After Effects is the Photoshop of video. Does many of the same things Photoshop does for a still photograph, but with an algorithmic understanding of a series of frames instead of a single frame. IOW, you define how long the effect lasts and it applies the effect to the underlying clip(s) for that length of time -- as a layer on top of the clips.
I'm not sure, in After Effects, how you could get an effect to work on a single frame but I supposed you could. If you did I'm not sure how you'd see it since it would go by in 1/24 of a second or faster. Which is why I wanted to confirm that we're talking about After Effects.