This doesn't work because for any given frame in the dash, the player is only in one of 4 discrete positions. All it does is increase the density at those points.
Since the particle system is a child of the player and has the same position, it's emission will only be from those 4 points, no matter how many particles are released.
For example, the position value of the player may look like this during walking (with x's as values):
-x-x-x-x-x-x-x-x-x-x-x-x-x-x-x-x-x
And like this during a dash:
x------------x--------------x--------------x