It should also work for those on an L visa, or am I wrong?
Never had an L or F visa, only a Z visa.

If a Yuming FT came to China on a Z visa Helen would have eventually changed that to a Residence Permit [Yuming keep your Expert's Certificate]- the problem with that is if he/she changed employer Yuming would give grief by refusing to provide a release letter; although that's not always the case, I managed to suggest a school to an American friend two weeks ago who couldn't get said letter-he was taken on because the school has clout with officials.

If the FT came with an L then Yuming would have changed it to an F which means he's not officially employed as a teacher anyway. Yuming would get him the F visa saying he will give lectures for the company; although, all the world and his mother seem to teach with an F visa- they almost stopped that a few years ago but now the F is being used a lot again on account of teacher shortages.

Essentially if the job depends on F visa which keeps need renewing you ain't lost much if you scarper. Yuming FT's who don't fancy 2-3 months unpaid may as well sod off to Thailand for a few weeks-when on holiday spend a few hours in internet cafe...easy peasy find another gig and return afresh with new L visa. Or just get on a train to Hong Kong. Now all that costs a little cash but surely you haven't come to Asia to sit and wait in miserable Tianjin apartment for the moment 2-3 months away when Helen deigns to offer you a class-or doesn't as the case could be- in that instance you may really be in trouble.