How a bol is physically produced: which finger, striking where, with what pressure, released how. The same written bol can have a different nikas in different gharanas — Delhi's two-finger Na against a Purab full-hand approach — and the difference is audible as tone.
Nikas is the first thing corrected in talim and the last thing mastered. Working one stroke slowly, listening only to tone quality, is standard method; repertoire played with poor nikas is considered unfinished regardless of speed.