An athlete's capacity to narrowly focus attention in a quality manner is harmed by too much mental energy.
When there isn't enough, irrelevant and important cues compete for attention.
Distractibility is a result of high levels od mental energy.
To get into the zone and reduce the amount of conscious internal processing, your attention must be at a level where performance can occur on an automatic basis.
Anything that requires the athlete to concentrate takes them out of the zone.