Three acts describe a script after it works. Eight sequences help you build one that does. The approach comes from the reel-length units of early cinema: roughly ten to fifteen pages each, every one with its own goal, complication and outcome — a little movie with a little ending.
The advantage is psychological as much as structural. A shapeless middle is unbearable to write because there is nothing to aim at; eight sequences give you eight finish lines, and each finish line hands you the tension that opens the next.
Each sequence should answer a question the previous one raised and raise a harder one. If two adjacent sequences answer the same question, they are one sequence wearing two hats, and you have found twelve pages to cut.
This is a diagnostic tool, not a formula. You are not obligated to hit page numbers. You are obligated to be able to say, for each block of your script, what the character was trying to do and how it ended.