Just like Fight Club and Who Dares, this session is all about surging well above threshold, and then trying to recover without dropping your power anywhere close to your recovery zone. The 2 things that make this session stand out are: 1. You only have one single continuous effort (which happens to be 45-minutes in length) 2. Your recovery power is low Tempo, which is good because any higher than that and you wouldn't last the full 60 minutes! As you increase the intensity from 80% of FTP up to 100% of FTP, your legs begin to produce more and more metabolites, but the rate at which your body CAN clear them is almost the same. As you close in on 100% of FTP you suddenly are at a point where the production of metabolites equals your clearance rate. Now move just a bit over FTP and those metabolites are going to continue to build until you drop your effort back below FTP. To improve your body's clearance rate for those metabolites, you need to spend a decent amount of time with more of those metabolites in your system than your body's current clearance rate can handle. That is what this session is all about, short hard efforts above FTP to build an excess of metabolites, followed by steady riding at 80% of FTP. This steady-state effort between surges is high enough that your body will still be working at max clearance rates but low enough that you can keep doing those surges over and over again, which is good for you because this is not a short interval. While you can achieve the same training stimuli by holding shorter blocks above FTP, those sessions have their drawbacks. The main one is nailing the exact right intensity above FTP that will allow you to accumulate enough time in that metabolite flooded state. While we like to think our FTP is set in stone, the reality is it can fluctuate from day to day, which makes finding that balancing point for a session like that so difficult. You can easily go too hard and blow up early, or not go hard enough and never really push that clearance rate to the max. By hitting surges well above FTP, then holding an effort a good deal below FTP, any day to day variation in your FTP becomes less of an issue because you never have to dance 5W above or below it. The added benefit to this Surge and Settle workout style is the boost it can give to your Anaerobic Capacity. An improved AC means you can recover faster, not just from sprint type efforts, but from repeated MAP and FTP efforts as well.