Specifically what does this mode/groove do?
It’s a combination of the Speed and Power modes with a few important changes.
First Meter Building is Slow.
Second you take slightly more damage.
Third you deal slightly less damage.
However you have access to everything.
Speed Chains, Power Unblockable, Can do Speed Combo with meter, can do Power Super with meter, can do desperation attacks when low on health and have meter.
It’s basically a best of both but with some drawbacks.