The role of the midwife when using TENS Machines

nothing, to faciltate it - some will put them on, but it's recommended you get your birth partner to do so, you may take them off but it's not covered in training etc

TENS is a self help method

It was covered in my training, I have a certificate to say I an competent to apply an obstetric TENS. Women usually come in with them on already though.

The role of the midwife with any non pharm pain relief iss imo crucial as any undermining of the pain relieving abilities of the method will interfere as it is hugely dependant on belief/perception, not saying there is no physiological basis but pain and pain relief are as much about perception/psychology. A good way to do this is to be able to explain how it works as people often ask, especially partners, even if they are already using it when they come in.