In order to use your momentum, you need to press Square just before your attack is complete. It’s best not to spam click Square, and it works a lot better if you just time it out and hit it once, you have a pretty forgiving window.
How do you use momentum mode in lost Sphear?
When you perform an attack, your character will have a blue glow above their head for a second or two before performing the attack. When you see this, just hit Square (or spam it 3-4 times, like I do.). This’ll activate momentum mode.