What trick did Penelope use to hold off the suitors?

She said she would choose one of them as a husband when she had finished the tapestry she was working on. But at night she unpicked many of the stitches she had done that day.

The tapestry was a pall (covering for a dead body) for Odysseus' father Laertes.
She asked them to wait for her to finish weaving a tapestry for her husband's father. She'd weave by day then unpick the loom at night.