The eve wiki says (Skills:Shields - EVE University Wiki):
“When your shields fall below 25% hitpoints damage begins to leak through to armor. This skill stops that happening. The exact mechanism of the ‘leaking’, and the consequent use or uselessness of Tactical Shield Manipulation, are subject to pedantic and rather irrelevant arguments; you need this skill trained to IV to use T2 active shield hardeners, so train it to IV and then forget about it.”
Why would they think damage not leaking through to armor wouldn’t be useful to a shield tanked ship??
Probably because it’s very situational and a hard to observe effect. Furthermore, the note is most likely aimed at newer players who could think that this skill us extremely important because it prevents your shields from breaking and leaking damage to your armor, when in reality that doesn’t happen too much anyway. Hence, you do not need to train this very long skill to V right away and get the best effect for most scenarios at level IV until you reach a point in your EVE career when you run out of useful things to train.
I think it goes like this:
If there was zero leakage, then the shields would go to zero before leaking to armor.
While this sounds like the best option in theory, there is another way to look at it:
The peak shield recharge rate is at a shield level that is above zero. This means that as you pass that spot and approach zero shields, the recharge rate gets somewhat worse than that peak level.
This means that in theory, if you let a little damage leak through into your armor, your shields are recharging better than they would be if they were at zero with no leakage. This extra recharge means more HP and shield resists to offset incoming damage.
With small ships of course the difference is so small it wouldn’t make a difference. Larger ships with huge shield HP the difference might be more noticeable.
correct, it is at the 15%-28% tile range, anything above or below this is a worse natural shield recharge rate…so those passive tanks that rely on passive shield recharge rates…