The words enim, etenim and neque enim are all postpositive tools for emphasis. They typically occur in the second position, but may occur in the third, where the second word is emphatic.
Enim and its emphatic counterpart, etenim, may add an affirmative pulse to a statement or clause. In this sense, they function much as equidem, certē and vērō, though these are not necessarily postpositive.
…sed enim istaec captio est: …but this is clearly a trick!
in hīs est enim aliqua obscūritās: in fact, these matters contain some mystery.
Quid agis?—Nihil enim: What are you up to?—Nothing, truly! (~Nothing, I swear!)
Etenim is very popular in parenthetical phrases.
dux huius agminis Caesar est (etenim est prīmus mīlitum): the leader of this line is Caesar, because, of course, he is the first among soldiers.
Kate Medoppidum (quae etenim modo hērēdem peperit) nōn trēs diēs vīsaest: Kate Middleton, who as you know just gave birth to the heir, has not been seen for three days.