Saber Dash Cancel

Zero's most effective damage technique(s) involve slashing, then dashing to interrupt the combo, then slashing again.

The first 2 hits of Zero's triple slash do not give bosses invincibility frames, allowing you to do this over and over again. It is a small amount of damage, but significantly faster than most other techniques.

Execution

In principle, execution is easy to explain - push dash, push slash, push dash, push slash. In practice, this can be slightly tricky to get right. 

One factor is where is your dash button? Many runners change the default controls, so dash can be in an awkward place to "double mash". The way you grip the controller during this mash can also make it hard to move otherwise.

You can also use double tap dash itself, though that works better with saber-saber-dash-cancel.

The other big factor is timing, making sure your dash actually interrupts a combo, and that the combo actually lands hits between dashes

Pegasus Ruins Things

In Mega Man X5, defeating Pegusus will give Zero a new attack executed by dashing and slashing. This negates this technique, and is why Pegasus is done next-to-last in all stages.