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Disruption Bombardment: In your Shooting phase, after
this unit has shot, if an enemy Infantry unit was hit by one
or more of those attacks made by this unit’s bio-cannons,
until the end of your opponent’s next turn, that enemy unit is
disrupted. While a unit is disrupted, subtract 2 from its Move
characteristic, and subtract 2 from Advance and Charge rolls
made for it.

So if they roll a 1 on an advance dice, do they move their base movement -2, or their base movement -3?

Rules Commentary - a dice roll can never be modified below 1.

So the answer is -1

-2 to base move
+1 from minimum Advance roll

That tracks. Thanks.