I've brought over some hair tonic.

First you head out to the cold outdoors.

Then hold a knife above the decanter, with the blade pointing down.

Dribble juuust

a tiny smidgen of hair tonic on top,

and the water and fragrances will freeze when they touch the knife,

allowing only alcohol to flow down into the decanter.

You don't have to do that even; I have alcohol here.

We drink them warm instead of chilled in Japan, yeah. Presumably meaning sake (traditionally served warm), and not, well, hair tonic.