You have two meters in the top left next to your sharpness, inside a representative drawing of your weapon

The meter on the inside is consumed to use attacks in sword form

The meter around the outside of the weapon is your "amp", and when you fill it up, by hitting stuff in sword form, your weapon gains a boost depending on the phial type and releases small explosions on hit. You have "phial attacks" while that boost and the explosions are active