Hi,
Reading an article on the BBC website, the following sentence caught my attention

PQR fears the future will not bode well for former dancing girls, some of whom have been forced into XYZ because hardliners objected to them performing relatively innocuous dances in public.

Now I know this sentence follows correct grammar, but can anyone highlight what exactly is the grammatical principle behind the highlighted part? Is 'performing' a gerund? Why is 'them' the best pronoun choice here, why not 'their' ?