Frank, I'm a happy snapper. When it comes to manipulating light and perspective, Kristina is the true genius. I just try to empulate what I learn by watching her in her art and emulate what I learn into my own style.
Armando, prefocusing on the log would not give very good results. It's a mix of both. I studied Amy, and how she jumps over obstacles for a while now. I know where during the jump she looks the way I want her to, so I take the log as a reference point, prefocus slightly in front of the log (about 30cm) because i know she will hover in about this position right there, then wait for her to be at that position. I start shooting about 3 frames before she traverses this point, and from the 4th frame onwards try to adjust focus to track the movement. This usually gives me 2-3 keepers in a series of 6 or 7 images.
When I'm lazy I just prefocus on a spot in front of the log and make Amy repeat the jump a few times