Sorry I didn't make this clear. So firstly, I ought to just add that you can only do the DAC with two controller schemes: the Gamecube controller (which is the one I was referring to previously) and the Wii+Chuck configuration. For the Wiichuck, you have to turn on the Shake Smash option.
Now, on with my explanation for the GCN controller.
You've noticed, I assume, how in certain situations the grab buttons and the attack buttons seem to overlap in function? (Meaning sometimes you will press Z and get an attack, and when you press A while in a shield you will grab)
Technically, the DAC can be performed by dashing, pressing the C-stick down, and pressing up on A or Z. But no one really has the reaction time or movement speed to hit the A button rather than the Z button, although technically doing a dash attack would be more effective than a grab if you messed the DAC up.
By switching L or R to attack, you dash, press down on the C-Stick, and then Up+R/L. Its automatically easier to hit the shoulder buttons on time rather than the Z button, when you do whiff (which is far rarer) you use a dash attack instead of a useless, laggy grab, and you've used a previously useless button to do it.
As a side note, this also makes bombsmashing way easier as well. You can still shield with the other shoulder button, as well as jump with the X and Y buttons and use the B button normally while you hold L/R.