This workout is similar to a very common FTP building working, the 2x20's, but with a twist. By changing the threshold blocks to Over/Under intervals you may also help train your body to recover when effort goes just below your threshold power.
Pro cyclist Mat Hayman proved that indoor training worked when he won Paris-Roubaix—one of the biggest races in the world—after breaking his arm earlier that year. This workout is how he did it. Note that this workout is made for a pro. If it's too difficult, try completing just one of the interval blocks and skipping the second.
Sweet Spot Training (or SST for short) will give great bang for buck as you can spend quite a bit of time in the Sweet Spot of your power curve without building up undue amounts of training stress. You can do this workout as much as you'd like and watch your FTP rise over time.