monstrum horrendum, informe, ingens, cui lūmen ademptumwhich, as he says, has to be scanned as
monstr’ ’or|rend’, in|form’, in|gens, cui | lūmen a|demptumto form a regular hexameter (six feet, namely four spondees, a dactyl, and a spondee; caesura in the third foot).
Another example: the third line of the Aeneid is
lītora,| mult(um) il|l(e) et ter|rīs iac|tātus et | altōwith two elisions as shown.
I am one of that dying breed: people who spent many hours as a teenager having to compose Latin and Greek verse. By the time I was sixteen I knew the rules of elision and hiatus avoidance, and would apply them in my weekly task of putting English poetry into Latin hexameters (or sometimes elegiac couplets) and into Greek iambic pentameters. This is a skill I can say I have since lost.
Looking now at my copy of Kennedy’s Revised Latin Primer, I am surprised to see that the whole matter is disposed of in a single paragraph.
(At my school we called it neither Synaloepha nor Ecthlipsis, just elision. This is where the use of the term “elision” in English phonetics comes from.)
The reason that a vowel plus m was subject to elision was that the spelling m here did not stand for any actual nasal consonant but just for nasalization of the vowel. So monstrum was pronounced mõ:strũ: (and ends up in Italian as mostro). The nasalized vowel, like any other vowel, would be in hiatus if immediately followed by another vowel and would normally be elided in this context.
In Latin, elision is not usually shown in writing (except sometimes in inscriptions, where you can get things like scriptust = scriptum est). This is different from the convention for classical Greek, in which elision is regularly shown by removal of the letters standing for deleted vowels, in modern texts with an added apostrophe to show the loss. Only short vowels could be elided in Greek.
Here’s what it says in my Sidgwick and Morice, An Introduction to Greek Verse Composition.