A provision in an ESPP with a longer offering period (e.g. 24 months) that has set purchase periods (every six months). If the stock price has decreased on the purchase date, an automatic rollover occurs that cancels the remaining purchase periods in the offering and re-enrolls employees in a new offering based on the lower start-date price.
Automatic rollover is a feature of what is sometimes informally termed a Cadillac ESPP (for more on that type of ESPP, see an article from the consulting firm Infinite Equity).
The feature is similar in some ways to an automatic reset. By comparison, a reset feature lowers the lookback price, but does not start a new offering period.