I have a few explanations.
1. The multiple uses of this "technique" in that video are a result of clever video editing. Notice we don't the percents, and pay close attention to the camera movement.
2. Turbo Mode is glitchy as balls, nothing that can be done in that mode makes sense or matters at all.
3. If you cancel an FSmash into a lower KB move the knockback doesn't stack and you end up with the KB of the last move you used. You are probably inadvertently cancelling the move into something else, or it's an inconsistency that you are witnessing because Turbo Mode is stupid.
TL;DR
This isn't a "technique" and there is no name for it. And the use of Ike's DSmash in that video is either edited in post, or he is cancelling the downsmash into a walk and then immediately doing it again.