The spirit comes from the still in three phases.
1. Foreshots. This is usually about the first half an hour of the run. This is high in alcohol but contains some undesirable alcohols and esters and is pretty much poisonous.
2. Middle cut. This is the desirable part of the distillate containing the compounds that will partly shape the whisky's flavour in the future.
3. Feints. Towards the end of the run further compounds, long chain alcohols and fusel oils, creep into the distillate. In high concentration these have an unpleasant flavour.
One of the many skills of the stillman is to judge when these phases begin and end, essentially what goes into barrells, and what goes back into the still for the next run.
Typical feints flavours are plastic, cheesy, soapy, musty and sweaty which are unpleasant additions to a whisky's flavour. Other feint flavours can be pleasant in small concentrations though, tobacco, leather, tea.
The timing of the cut, and the amount of these flavours that make it into the spirit, are one of the many variables that make up the character of the finished whisky.