In GA, a fortis stop is inserted in words like nymph, sense, strength. If a nasal and a voiceless fricative occur in the coda, a fortis stop is inserted which has the same place of articulation as the nasal. For the above examples, the result is /nɪmpf, sɛnts, strɛŋkθ/.
The fricative can also be taken into the coda as a result of the Weak Syllable Principle. That is why stop insertion also occurs in symphony, concert, lengthen, where the voiceless fricative is ambisyllabic. The result is /ˈsɪmpfəni, ˈkɑːntsərt, ˈlɛŋkθən/.
Recall that the WSP does not apply when the following vowel is strong: consort /ˈkɑːnsɔrt/, sensation, Bloomfield have no stop insertion.
Stop insertion arises as a result of a ‘premature’ raising of the lowered soft palate in anticipation of the velic closure required for the following oral fricative. When the velic closure precedes the release of the oral closure for the nasal, a stop is produced.
When words like symphony, sense and lengthen have undergone stop insertion, they are subject to nasalization and nasal deletion, just like camp, cant, bank (which, you will recall, are pronounced [kʰæ̃p͜ʔ, kʰæ̃t͜ʔ, bæ̃k͜ʔ]). So sense may be [sɛ̃t͜ʔs] and strength [strɛ̃k͜ʔθ].
Advice for Dutch learners
Stop insertion leads to homophony in pairs like prince – prints, tense – tents and sense – cents. In proper names like Thompson and Sampson as well as in words like empty and contempt the inserted stop has made its way into the spelling.
Unlike n-deletion in Dutch, which tends to apply before consonants other than stops, as in dans, mensen, English nasal deletion applies before fortis stops only, including inserted stops. A word like dance is therefore realized as [d̥æ̃t͜ʔs] rather than [d̥æ̃s]. It may therefore be a good idea to concentrate on pronouncing a (glottalized) stop in words like concert, sensitive, defense, chance, difference, thanks rather than on deletion of the nasal.
