Magic & Tech
The duality of Spells and Tech is heavily studied, but it is generally agreed they interact with the body through the same way.
They use up naturally generated mana within the body, and channel it to create the desired effect.
Each person has a limited amount of mana before it has to be regenerated, this is represented by Strain. The more spells you cast, the higher your Strain, and the higher your Strain, the less effective your Spells become.
It works similarly for Tech, each item has a Strain Cap, which is permanently added to your strain, and each tech item has a flaw, which activates when its Tech Cap is reached, this is used to signify when your bandwidth is low and the accessory is not receiving enough mana.
Your strain, while it does have a maximum, it only functions as a soft maximum, you can surpass it, but at the cost of debuffs. Check Strain for more details.