It was the concept of filling out the volumes with short speed to keep the total weekly volumes constant throughout the main body of the SPP until the very last minute. Up till then he (and most other coaches) had simply followed the pattern of L-to-S SE work, allowing the volumes to drop by the amounts removed with each shortening of the SE distances. this resulted in a very narrow peak as too much work was removed for too long to maintain fitness for a sufficient period.
For each individual, the SE distances and the duration of the phase will be different. Both the 100/200, and 400 L-to-S charts I’ve presented were created for specific individuals. The 400 L-to-S and S-to-L charts were posted free on the forum recently if you haven’t seen them yet.