Just seconding what Guest above said. According to Betty Messenger's excellent book Picking up the Linen Threads, hackling consisted of drawing handfuls of previously scutched flax fibers through a series of hackles, i.e. tools consisting of rows of sharp, pointed pins set into a board, with the goal of cleaning and squaring up the pieces and splitting the fibers into finer filaments of an even size ready for further processing.
In informal mill parlance its practitioners were known as hacklers, flax dressers or sorters.
They had their own songs, e.g. The Hackler from Grouse Hall or:
Hi ho, the hacklers-o,
Hi ho, the hacklers-o.
Hi ho, the hacklers-o.
On a Monday morning.

There they go,
with their long-tailed coats,
Silken hats and dandy cuffs,
Their aprons shine like powder puffs
All on a Monday morning.

Seamus