The second bite is the founder's proceeds from the sale of rollover equity when the buyer exits.
In a sale
With a capable buyer the second bite can exceed the first sale; with the wrong one it can be worth nothing. It is paid when the buyer sells the business again, on the stake the founder kept. That range is why the size of the roll, and the choice of buyer, matter more than the promise of a second sale.
Where it comes up
The Go-To-Market Analysis (GTMA) builds the file a buyer would build on your business, before a buyer does.