Greetings Chris, when you say you shoot N to N-6, do you mean ND Graduated Filters ? Sorry for this question,
but Im really new to this and would like to learn....
No, Chris meant that he varied the exposure in successive frames, by shortening the shutter speed, from the "Normal" (N, the "recommended" metered speed for the overall scene), to 1 stop less, 2 stops less, ...down to 6 stops less (N-6).
The "N+/-X" terminology comes from deliberate under/over-exposure of film and compensating under/over-development - especially large format sheet film shooters using the zone system.