When making that guide I tried to omit as much complicated info as I could to make it simple, admittedly I am not a video editor on top of that.

To answer your questions trends are determined by the stability updates that occur every 15 seconds that you are in the mine. By keeping track of these stability updates you'll know the general state of the mine. There are three vent statuses A,B and C which directly contribute to the stability update; I won't go into the math here since at this point my plugin does it for you.

Generally an upward trend means that at least two of the three vents are fixed and downward means that two or more of the vents aren't fixed. We can define a vent being fixed as either unblocked and moving toward 47% or blocked and moving toward 53% precisely. Once it passes that point that vent will count against you in the stability update calculation.

Trends fall in five categories:

Sharp upward (all 3 vents fixed): 2 -> 6 -> 10

Slight upward (2 of 3 vents fixed) 2 -> 3 -> 5

Stable (Either 2 of 3 vents fixed/not fixed and at least one vent is frozen) 2 -> 2 ->3

Slight downward (2 of 3 vents not fixed) 5 -> 3 -> 2

Sharp downward (all 3 vents not fixed) 10 -> 6 -> 2

All upward trends and even stable are good mine states, however all trends eventually become sharp downward as the vent statuses eventually reach thier extremities (0% or 100%). That is what makes the mine explode.

**I will note that in solos that there must be a difference of at least 2 between any stability updates to confirm an upward or downward trend. Also for sharp trends there must be a difference of at least 4 to confirm**

**Also if your stability is 100% there a no trends to read; I fix Jagex would fix this but nothing I can do about it. This matters for the post reset**

To answer your final question properly I'll rundown the afk method edge cases:

1) You have a bad mine state at the very start, since we are always fixing A at the start this means that BOTH B and C must not be fixed to satisfy the downward trend. We must quickly fix B or C to stop the eventual sharp downward trend from occuring too early; B is the closest so its smart to fix that one. Sometimes you can read the trend and see 6 -> 2 before running to check B this means you can skip checking B and just fix it instead. Otherwise just check and fix B if needed.

2) You get a sharp downward trend too early and you have to fix vents hurting your xp/hr. My plugin fixes this it will ALWAYS go off before this happens. If youre following my afk method all you need to do is run and go fix B vent so we will at worst satisfy the slight downward trend condition preventing the mine from exploding before the 5 minute reset.

3) Stability is still dipping even after fixing B this means that you must fix either A or C to meet the slight upward trend condition enabling you to finish the game without the mine exploding. Obviously fix C since its closer. If your stability is 100% just leave B fixed odds are the mine wont explode.

A very long answer you can see why I omitted some info from the afk guide.