The general opinion is that they are the most effective at 40+10. I went for one at 40+8 and she couldn't even reach my cervix. I found it quite uncomfortable when they tried it again in hospital before they started my induction but not unbearable. I would have it done again if needed.
the mw basically puts her fingers inside you, finds you cervix and if possible slips a finger in and sweeps round the inside to separate the sac from the wall (hope that makes sense). it didnt hurt me just remember to keep your body calm xx
i got mine today.. really nervous acctually.. 41 weeks.. fingers crossed it works!