From SmashWiki, the Super Smash Bros. wiki
The second custom variation of Dancing Blade is called Heavy Blade. Each swing of this attack has significantly more range, damage and knockback, in exchange for stricter input timing and slightly longer startup. Due to both the tighter input window and the greater knockback, its hits are much harder to link together, and because of this Heavy Blade is generally considered inferior to Marth's default side special. This is exacerbated by it being highly punishable if blocked or avoided and being a largely unreliable KO move despite its power.