I'm not near my game right now, so you may want to take this with a grain of salt, but from what I can picture in my brain, you should start your dash when they are at their peak height if the bounce. I don't know if that makes sense or not, but when they di inward, you can wait for the smallest of fractions of seconds, until they hit their max height and then dash and jump cancel grab. When they di outward, you need to start your dash as soon as possible. Once you start getting the feel for it you can tell when you're late on the dash and that's when you know you need to boost grab. If that doesn't make sense I'll try to explain it better, but I'm in class right now >.>