How does Thornblade gain Sword Dance stacks?

The loop is short. Fire a skill, then cash the stack into your basic attacks. One skill use grants one stack of Sword Dance, so the rate at which you empower your attacks is set entirely by how often your skills come up, not by how fast you swing.

That has a clear consequence for how you play. A Thornblade that stands there chaining basic attacks is running unempowered, because nothing about a normal attack feeds the counter. Skills are the only input. Sitting on a skill that is off cooldown is the same as sitting on damage you have not switched on yet, so the rhythm you want is skill into attacks, then the next skill as soon as it is back.

Twinning is where this matters most. It replaces your standard mobility, attacks and skills with direct manual control over the Aniimo's movement, combos and abilities, which means the stack cycle is something you drive by hand, and the spacing between skill uses becomes your decision rather than something that happens on its own.

Not settled: How stacks are gained is settled, but how many Sword Dance holds at once, whether the stacks expire on a timer, and how much a single stack adds to an empowered basic attack are not.