Exegesis:

kai diēgēsanto autois hoi idontes ‘and the eye-witnesses described to them.’

diēgeomai (9.8) ‘relate,’ ‘describe,’ ‘narrate,’ ‘explain.’

pōs egeneto tō daimonizomenō ‘what had happened to the demon-possessed man’: pōs usually means ‘how’ (cf. 2.26) and could be so translated here ‘how it happened to the demon-possessed man’; it may mean, as it probably does here, ‘what’ (Revised Standard Version, Translator’s New Testament.

kai peri tōn choirōn ‘and (also) about the pigs’: additional explanation supplied by the eye-witnesses.

tōn horiōn autōn ‘their regions’: the word horiōn (7.24, 31) ‘boundary,’ ‘limit’ in the N.T. is always used in the plural ‘boundaries,’ with the resultant meaning of the region encompassed by the boundaries – ‘region,’ ‘district,’ ‘province’ .

Translation:

The syntax of verse 16 is complicated by the double grammatical reference to the same event through the use of it and what had happened to the demoniac and to the swine. These eyewitnesses simply reported what they had seen. This means that in some languages the descriptive statement ‘what happened to the demoniac and the swine’ must be attached to the verb of ‘seeing,’ e.g. ‘those who had seen what happened to the demoniac and to the swine told this to the people.’

As in verse 15, the translation of demoniac must not indicate his continued possession.

They of verse 17 refers to the people in general, not just to the eyewitnesses specified in verse 16. That is to say, after the people heard what had happened, they asked Jesus to depart out of their country or territory.

Quoted with permission from Bratcher, Robert G. and Nida, Eugene A. A Handbook on the Gospel of Mark. (UBS Handbook Series). New York: UBS, 1961. For this and other handbooks for translators see here .