Halliday & Matthiessen (2014: 467):
In written English, a non-defining relative clause is marked off by punctuation — usually commas, but sometimes by being introduced with a dash; whereas a defining relative clause is not separated by punctuation from its antecedent. This in turn reflects the the fact that in spoken English, whereas the defining relative clause enters into a single tone group with its antecedent, a non-defining relative clause forms a separate tone group. Furthermore, the primary and secondary clauses are linked by tone concord: that is to say, they are spoken on the same tone. …
Whichever tone is used, however, it will be the same in both parts; the tone selected for the (relevant portion of the) primary clause is repeated in the secondary clause. This tone concord is the principal signal of the apposition relationship in English, and applies also to paratactic clause complexes of exposition and exemplification referred to above (though not to clarification, where there is greater semantic distance between the primary and secondary clause). We should also note that this pattern is very frequently accompanied by a rhythmic feature, by which the secondary clause is introduced by a silent beat.