When pitches go astray, catchers become blockers. You always want to be positioned square to the pitch, so any deflection just trickles back to the plate. First challenge is to know where the pitch is going. Expectations would have a topspin curveball cutting back and up, and a red-dot slider slicing low and away. Everything happens almost together, but get your hands down first, then your knees, shoulders and chin tuck.
Knees drop second.
If you were in proper blocking stance with ankles outside hips, the knees should come straight down either side of mitt. Remember you aren't trying to catch the ball. Kicking the leg out first in the direction of bounce. Upper body first - if the feet are still firmly planted, you will get limited lateral movement. Hips first - we like this approach.