The first time on your turn when you use the Sustain action to maintain an Infusion, it loses the Manipulate tag and you can perform it as a Free Action instead of a Minor Action. This allows you to maintain at least one Infusion during Discovery and Downtime without strain.
For each rank after the first you have of this ability, you can maintain +1 additional Infusion with each use of the Sustain action. For instance, with 2 ranks of this ability, if you use the Sustain action twice, you can maintain 4 different Infusions.