Step 1: Think about what a solidifying casting needs from its mould.
As molten metal cools inside a sand mould, it shrinks, and if the surrounding sand grains refuse to move out of the way, that shrinkage gets resisted, building up stress inside the still weak, newly solidified metal.
Step 2: See what wood flour does when it gets hot.
Wood flour is essentially fine organic material mixed into the sand, and when the hot metal is poured, this organic material burns away and leaves tiny voids scattered through the sand mass around the casting.
Step 3: Connect those voids to the mould's behaviour.
With these small voids present, the sand grains have room to shift and give way as the casting contracts, so the mould collapses or yields easily instead of clamping down rigidly on the shrinking metal. This yielding ability is exactly what is meant by collapsibility, and it is the main reason wood flour is added, helping to prevent shrinkage related defects like hot tears.
\[ \boxed{\text{collapsibility}} \]