Mindfulness is not a technique so much as an important aspect of magical practice.
Mindfulness is being in the moment, allowing whatever task you’re performing to have the whole of your attention in a relaxed fashion, so that you are fully open to the experience.
Mindfulness is important in magic because you are tied to your environment, which impacts you.
Being aware of everything is critical to being able to pick and choose what you want to draw on in your magical work.
It’s also important because you need to be able to differentiate between good and bad (or supportive and unsupportive) energy.
Reaching out and being able to sense what is around you takes a curious blend of concentration and release.