Viscimation is the turbulence when liquids of different viscosities mix, particularly the formation of vortices (called "viscimetric whorls") and visible separate threads of the different liquids.
The term is archaic and idiosyncratic to whisky tasting; their study (or appreciation) is called viscimetry, and the capacity of a whisky to sustain viscimation (which is predominantly its alcohol percentage) is called viscimetric potential or viscimetric index.
Causing viscimetric whorls by adding water to liquor is called "awakening the serpent".
Not to be confused with viscometry.
References
Viscimetry Wikipedia(Text) CC BY-SA