They don't go into any detail on the sites you link to and I don't have time to look into patents, but from the context it sounds like they may be using a tetracycline repressible promoter. This is often referred to as a Tet-Off promoter.
The core elements of these promoters were discovered in the bacterium Escherichia coli1 and several variations have been ...
The other answers are correct and on the right track, but I will expand on them with the specific mechanism for these mosquitos. The following paper discusses the engineering behind strain OX513A upon which the OX5034 strain is based:
Phuc HK et al. 2007. Late-acting dominant lethal genetic systems and mosquito control. BMC Biol 5:11
Oxitec mosquitos carry a ...
Males are effective sterilization vectors
You can think of male mosquitos as fertile only in theory. Any mating of these mosquitos will generate no offspring whatsoever, so they are effective sterile and they spread this characteristic to females.
The bacteria makes they sterile in the end, the same way any disease in others cases.