It is governed by the action of the spinnerets, which contract or expand their pores, or close them entirely, at the faller's pleasure.

And so, with gentle moderation, she pays out this living plumb-line, of which my lantern clearly shows me the plumb, but not always the line.

The great squab seems at such times to be sprawling in space, without the least support.
She comes to an abrupt stop two inches from the ground; the silk-reel ceases working.

The Spider turns round, clutches the line which she has just obtained and climbs up by this road, still spinning.

But, this time, as she is no longer assisted by the force of gravity, the thread is extracted in another manner.