In a spin-off you wake up owning two tickers. The parent's price drops by roughly the value handed out, and your cost-basis is split between the two positions using a ratio the company publishes in an 8-K.
Spin-offs create predictable, mechanical selling. Index funds holding the parent often cannot hold the spun-off company, so they sell it in the first days regardless of value. That flow is the reason spin-offs are watched closely.
Example: a parent at $90 spins off one share of a subsidiary for every four held. The subsidiary opens at $32, so $8 of value per parent share leaves and the parent trades near $82. A 400-share holder receives 100 subsidiary shares.