Martin (1992: 193):
With [manner] relations, the relationship between events is modulated through "potentiality"; we won by training hard means that the Cause (preparing well) enabled the Effect (winning). With other consequential relations the connection between events is modulated through "obligation": we won because we trained hard means that the Cause determined the Effect. This is the "natural logic" of the distinction between sufficient and necessary conditions:
we won by training hard (among other things) :we trained hard enough to win (but we lost) ::
we won because we trained hard :we trained hard enough to make sure we won
 This confuses the ideational metafunction (expansion) with the interpersonal metafunction (modality).
 This is a fundamental category error. Manner is not cause and effect. In the clause complex, training hard is construed as the manner: means by which we won, but not as the cause. Manner and cause are semantically distinct. For example, the manner of the wind blowing is semantically distinct from the cause of the wind blowing.
 It is a category error to construe the distinction between manner and cause as the distinction between sufficient and necessary conditions. See yesterday's post or the discussion on necessity and sufficiency in formal logic here.