Regular boost Smashing requires you to do the Dash attack with Down C-Stick. Doing up + Z enables the charging Smash when performing Smash Attacks using the C-stick (like you would doing charged Smash attacks regularly). This is what cancels the Dash Attack animation which allows you to perform the boost smash.
Thats why regular boost smashing goes as follows:
Dash Attack(Down C-stick) => instantly press Up + Z
You can hold Z to charge up the smash attack while sliding to your enemy.
Gatling Boost Smash (chaining both the Dash Attack and the U-Smash) just requires you to U - Smash in anyway immediately upon the first few frames of the Dash Attack making contact with the enemy regardless of shielding(which makes it a good shield breaker even at higher percentages)