Patching is the practice of covering one eye to be able to give stimulus to the other eye, independently. This is most often applied to activate the non dominant eye (also see ocular dominance). If one eye is improving faster than the other, or if you started with a large gap in refractive state between the two eyes, you might patch the "good" eye and try to active focus with just the "bad" eye for short periods.
15 minute sessions a few times a week is sufficient, more than that is not recommended. It might be a good idea to alternate your sessions, one time in close up with differentials (if needed), then the next time during distance vision time with your normalized.
This technique is generally not recommended when first starting out with EM methods. This is an advanced technique frequently used during equalizing, when equalizing it is typically best to do a few spherical reductions first, though there can be exceptions.
Patching is also used in conventional optometry for vision training and for rest while an eye is healing from trauma.
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