"Do you mean from the young to the old?"

I'm thinking of wealth as the claim on productive assets such as stocks, bonds and property. Pensions are a structured transfer of wealth from the old to the young and of labor from the young to the old.

"I can't think of the mechanism..."

Inadequate savings can create structural problems that result in temporary mismatches between the desire to work and the desire to consume.

For example if retirees are worried that savings or pensions are inadequate they may delay consumption which can create temporary high unemployment.