That is a common misconception, as just by reading the text it would appear to work that way. Flame Burst augmented with the Bursting Flame and Sweltering Heat skills will not apply the Sweltering Heat slow consecutively for more than two times in a row.
To enable Sweltering Heat the ability to be applied again, you must either:
Wait for the your target to not have the CGC dot
Have another railshot refresh the CGC dot on your target (Superheated Rail).
Chilling Scream and Hamstring are examples of spammable perma slows.
i believe what was being referred to was that Powertech is able to keep their target perma-slowed through the use of Flame Burst, which also procs the CGC dot as well as procs Rail Shot resets.
either way, Missile Blast is a perfect candidate for a slow like this. it is very high heat cost, very low damage. meaning it cannot be spammed while maintaining effective dps, and that it cannot be spammed as a means to produce effective dps. spamming of Missile Blast would allow for a perma-slow effect, but would result in quickly overheating and significantly reduce damage output.