XP Training

[danr]

Okay, I know a skill trains when you use it, and if you turn it off, it has a reduced chance of training (1/4 in stable versions and something like 1/skill level in newer versions).

However, sometimes I use a skill or invocation, such as MP channelling, and none of my XP pool is used (esp. if I only have 1 XP in the pool). Is there some randomness to whether a skill gets trained even when turned "on"? Or is it a case of 1 XP not being sufficient to train that skill at all, so it just leaves it until there is more to work with?

Other times, I'll blow up a giant rat with magic dart, and I had 0 XP pool before, and 0 XP after. Did the 1 XP from killing the rat get trained to conjurations in the same "turn"? I'd like to know because it means that a skill can be trained with 0 XP pool if the skill gets the killing blow.

[galehar]

You mostly answered your own questions.
When you don't have enough XP in the pool to train a skill, it will just stay there. In 0.7, there was a chance that a magic school skill wouldn't train when you already know many of them. I think it included evo/invo. This obscure mechanism has been removed in 0.8.

When a skill is used to deal the killing blow, the XP gained is immediately used for training the skill. That's why the pure offensive skills are easier to train than the supportive ones (conj vs ench for example).