Sorry about that, we could've made it more obvious! The direction keys correspond to corners in the octagon - e.g., (you can rebind these, but WASD + Space for the sake of example) S is the bottom corner, W is the top, A is left and D is right. Combining these keys aims towards the diagonal corners, e.g, holding WA is the top left corner, SD is the bottom right, and so on. When you pick the corner you want to move to, you press space to dash there.
If you try with a mouse (just clicking on the corner you want to dash to) or try it on mobile (tapping/swiping), it might be a bit more intuitive :)