Assignment is the short option holder's side of exercise: the seller of a put buys 100 shares per contract at the strike, the seller of a call delivers them. United States equity options are American-style, so assignment can arrive any day, but it becomes likely only when the option's remaining time value falls below what the holder gains by exercising.
Exercising an option forfeits its remaining time value: a holder who exercises a put worth $5.00 with $0.40 of time value left has thrown away $0.40 versus selling the option and trading the stock directly. So rational early exercise concentrates where time value has drained: deep in-the-money options near expiration, short calls just before a dividend larger than the call's remaining time value, and short puts whose time value has fallen below the interest earned on the strike proceeds. A useful monitor flags exactly that comparison; Options Scanner stamps short in-the-money puts whose remaining time value has dropped below the exercise-interest threshold as assignment-likely, because past that line assignment is the holder's rational move.
Nothing dramatic, mechanically: the position statement swaps an option for stock at the strike, cash-settled overnight through the clearing house. For a cash-secured put the reserved cash buys the shares, which is the wheel's designed transition. For a spread, assignment on the short leg leaves the long leg still in place as protection: the combined position's worst case is unchanged, which is precisely why defined-risk structures treat assignment as an administrative event rather than an emergency. The follow-up is a decision, not a scramble: exercise the long leg, sell the shares, or hold them.
Pin risk is the expiration-day ambiguity of a short strike sitting at the money at the close: assignment depends on a counterparty's decision made after the market closes, so the position's overnight stock exposure is unknown. The clean answer is not owning that ambiguity: closing or rolling short strikes that are near the money into the final session, which the 21-DTE exit convention makes moot for most disciplined positions anyway.