All of these replays should (hopefully) show different aspects of decapping.
If you can get the aim perfect, you can decap even if you don't hit very hard. The back of the neck is usually the best place to aim for if you're trying to do this.
If you don't think you can get the decap right away, try to bruise up the neck a bit anyways, this makes decapping easier later.
Don't think only your arms can decap, anything can do it if you get creative enough.
Pushing the head out of the way can decap if you do it fast enough, slowly will usually frac though.
Try to use every joint you can to make the decap more powerful, every bit helps.
If you think you're going to hit really hard, you don't need to worry about aim too much.
Sudden changes like contracting a joint around the neck is usually a good way to get a decap.
Twisting the head around can make it pop off (try to grab the neck).
If you have a grab on the head but cant get to the neck, if you jerk it hard enough (har har) you might be able to decap. Try contracting your whole arm and then fully extending it. If that doesn't seem fast enough, contracting your wrist instead of extending it can make your arm move faster.
All of these should correlate with one of these replays.