Men of magic can syphon a fragment of the energy from these lines,
especially during certain periods and positions of the moon and stars.
The additional magic energy is often only accessibly directly at a ley line nexus
and must be used while it is available.
The PPE cannot be saved and used later.
I think I had some misconceptions about this because Rifts Ultimate Edition didn't exactly mention these prohibitions ("must be used" + "cannot be saved") so maybe they don't apply.
I used to think that the 10 PPE/melee could be used to replenish one's PPE stores, but I think I was wrong about that. I guess you'd have to use it quickly for casting a spell, though I'm not entirely sure how quickly. Like if you wanted to cast a 20 PPE spell you'd need to tap for 2 melees to cast without tapping your own, but is that possible, or can you only tap the 10 and it's gone if it's not used?
Is that maybe what the "3xPPE for PE in minutes/hours" I've seen in some errata refers to? I can't remember which (if either) is canon for PF2.
Being unable to recharge your base PPE using this 10/melee from being at/on a line (or half mile from nexus, if off a line) is probably why it separately lists your own PPE reserve replenishing at 10 per 30 minutes (without meditation!) which would seem pointless to do if you could get that much in 15 seconds.
As a house rule I'd probably like to simplify that to 1 per 3 minutes, to allow someone to dip their toe onto a line for shorter periods of replenishment. I think I'd also stack this with usual sleep/meditation rates (which could be legal, it never says to replace the usual rates so it might be viewed as a supplement).
I wonder about ways around this though. Talisman PPE batteries (pg 215) seem line one, since you could create those using the 10/melee PPE.
When it says "can then be used at a later time to bolster the character's normal PPE reserve" I don't know that it necessarily means you could recover your base PPE using them though, but rather it seems like they might function like a ley line where it's an excess you can use to boost spell-casting directly?
IE if you have 0/100 PPE and a pair of 50 PPE talismans which are full, I don't know if you could just suck out the PPE to replnish your base and then toss away the empty talismans, I think maybe you'd need to keep the talismans as a "bolster" for direct spellcasting? Correct me if I'm wrong but I guess that's how I view the word "bolster" and the lack of mention that you could explicitly recover from them.
Another comparison I'm wondering about with Rifts. RUE 186 says that mages can suck 10 PPE / melee from ley lines, which makes it sound like it's the same as PF. This seems to contradict PF2p280 which says "energy levels are roughly 60% less than Rifts Earth". It made me think that if PF has merely 40% of the energy that it should instead be 4/melee instead of 10/melee.
However I must keep in mind: Ley Line Walkers/Rifters can draw 20/round from a ley line (and I think Mystic Knights too, it notes they can draw double in Federation of Magic), so THAT should probably be viewed as the full level. Rifts has double the levels, but only Walkers/Rifters have the skill to actually tap it safely.
That said... 40% of 20 PPE is actually 8 PPE not 10 PPE, so shouldn't we actually be tweaking the PPE/round in PF2 to 8/melee?
The only other explanation I can think of is that 10 PPE is 40% of 25, so perhaps the PPE/round in Rifts at lines is actually 25 but nobody's skilled enough to grab it, Walkers/Rifters simply come closest at 20.