Four Dimensional Power Focus:
One of the few Sufferfest sessions that can be viewed as a “Sweet-Spot” session since the intervals aren’t above threshold (except for the very end) but are harder than a tempo effort.
This session pushes your body close to the limit of what it can 'comfortably' handle without going any further. Why? Going above threshold causes a considerable amount of hormonal stress that takes a lot of time to recover from. But you don’t actually have to ride above threshold to improve your FTP - the fitness gain between 30 minutes at 90% of threshold and at 102% of threshold is minimal, but the stress on your body from the later is much greater and the recovery much longer.
That is why this session is a key component to any training plan: It gives you enough training stress to help improve your FTP, but remains 'easy' enough that you can do it when you're tired or before a big block of harder efforts.