The choice to activate Zelda’s Swordfighter form is governed by a source identified simply just as Vitality, which it does not appear to be extremely plentiful, according to what we played. The things that Zelda can summon as echoes are governed by the power of her Tri Rod. From https://icelisting.com/story19256002/the-2-minute-rule-for-the-legend-of-zelda-echoes-of-wisdom-gameplay