[unwind]

Ah, this [1] meaning of tillering (bending wood to form a bow), not this [2] (production of side shoots in grasses). The joys of new words.

[1]: https://www.howtomakealongbow.co.uk/part-5-tillering

[2]: https://en.wikipedia.org/wiki/Tiller_(botany)

[defrost]

As I recall tillering is more about the shaping of the bow to achieve an optimal bend and force delivery on release.

It's an iterative process of bending and shaping, bending again, and wood removal in stages.