Byleth's Dash Attack is a heavier version of her f-tilt. Unlike f-tilt, Dash Attack has a sweet and sour spot. The sweet spot is on the sword, and the sour spot is on Byleth's body.
Dash attack is very fast, only slightly slower then f-tilt, but has much more endlag. In exchange, Dash Attack gets more KO power. Due to this, Dash Attack is of less use in neutral then f-tilt, but is much better as a combo ender.
The most prominent kill confirm with Dash Attack is Nairquake -> Dash Attack, which kills around 120%.