I think this means that, if the insured cancels his policy, the company will retain a portion of the premium already paid by the insured i.e. the insured will not get all his money back. This portion, however, will not be only a fraction of the long-term premium (i.e. it will not be the long-term premium multiplied by the fraction of the time the person was insured, which would then have been a pro-rated portion). Rather, they will now charge the insured the premium they would have charged if he had taken a short-term insurance. Which is obviously a higher rate.

"short rate
taux courte durée (JD)
short rate
taux court terme, taux période temporaire, taux coute durée (LGA) "
(see site below)

I would translate it with