You may become trained in special abilities
Abilities like Bash or Pierce may also be Trained or even Specialized on, as they are Special Abilities (Core Rulebook, pg.21).
Character types and foci grant PCs special abilities
at each new tier. Using these abilities usually costs
points from your stat Pools; the cost is listed in
parentheses after the ability name.
They work as normal, when using that ability, you reduce the difficulty by one step (or two if Specialized), as explained under Character Tiers and Benefits: Skills (Core Rulebook, pg.18):
The skill you choose
for this benefit can be anything you wish,
such as climbing, jumping, persuading,
or sneaking. You can also choose to be
knowledgeable in a certain area of lore, such
as history or geology. You can even choose
a skill based on your character’s special
abilities. For example, if your character can
make an Intellect roll to blast an enemy
with mental force, you can become trained
in using that ability, treating its difficulty as
one step lower than normal.
So, if you decide to become Trained in Bash, all your attacks using Bash will have the difficulty reduced by one step, but your other attacks will not be affected.
But you cannot become Trained in "attacks" or "defenses", as that is too general for the system.