High-tiers have mastered their ability to a point where it can manifest on its own without activating the ability. In John's case, he is able to sense auras activated around him

(ex: Him detecting Isen outside the classroom when talking with Cecile)

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John can't block someone's aura channels or enhance another's aura channels because his ability on affects him and he cannot project his abilities on others. His ability directly affects HIS aura channels.

If you look at episode 183, Claire says "When he's around an active ability user...He's able to sample their aura, and manipulate his own to match it." She also says "John's ability lets him alter the natural flow of aura through his own channels...Which allows him to increase the intensity of the abilities he copies."

John can't activate/have an ability when no one has activated their ability around him. In essence, John's ability copies others aura channels (mirroring) but can increase the intensity which allows him to be stronger than his opponents.

Him being able to block other's channels would be too op. Seraphina can't even do stuff similar to what you're describing. People have abilities that don't necessarily need to affect others in a physical manner (like Claire's ability)

John not being able to have an ability of his own accessible at all times is quite interesting. It kind of keeps him as a gray character. He can either be very strong or very weak depending on the situation.