jibboo,
You came to the right place, I wrote that gibberish, or at least was principal author, but by the time the tech pub writers finished.......
The basic idea of Power Assurance is to be sure the engine will let you meet published performance. The trend method allows some daily wandering of the margin, called scatter, which is assumed to be merely statistical and not actually indicating an engine problem. The rationale for each:
1) means = The engine meets the min specification power.
2) note: The "daily trend margin" means any individual daily power margin check. Means = No daily check can be very much out of whack - this spells trouble, maybe the engine has a real problem, and the result greater than 4% from the average is beyond scatter.
3) Means = if the margin at low power is less than the margin at high power, if you really had an OEI event, the engine might not give you 30 min and 2 1/2 min power. This covers an engine that has a "negative slope" which means it has less margin at high power than at low power.
Make any more sense?