OK, I've got a bit more understanding now. I didn't realise you could
bid more AP than you actually had, apparently the limit is your starting
AP (but just for heros). And the amount of AP bid modifies the scale of
the sub-contest, so bidding 8 AP to knock over the trooper wouldn't do
much - which gets storied by the narrator as nudging the trooper with
the cavalry soldier's horse, even with a critical success on the
sub-contest.