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That's the session, yes. Once or twice per week. Incremental improvement through the winter like Milo of Croton and his bull calf. Gain a watt or three per week. Week fifteen feels like the same effort as week 1.
But no FTP testing. The 2x20 is a weekly test. "Training is testing, testing is training". Innit.
Old hat now. Still seems very effective and I wouldn't expect to get fit without it.
is this just a series of workouts with ten min warm up/cool down and 2x20 intervals at 90% FTP, with 5 mins rest in-between? by 'incremental threshold improvement' I assume you mean you test your FTP regularly after a few weeks of doing these three times a week?
edit: seems that you should maybe do the 20s at 95-100% of FTP