As the others have said you need a specific labour TENS machine with a boost button, you can hire them for about £25.
I was terrified of the idea of using it but I was planning a home birth I wanted as many pain relief options available in my arsenal so to speak. After a while the MW and my H convinced me to try it, I was terrified of pressing the boost button at first, but in actual fact it did work. I think partly it was taking my mind of the contraction as it gave me something else to focus on and I was actually able to sleep during contractions whilst using it. I still have lovely memories of H and I curled up on the bed with me dosing off between the contractions whilst he cuddled me.
I have no idea where the pads went, I don't even remember them being put on, so I guess either H or the midwife did it.
Apologies for lack of capital letters - Zoe has stolen my shift key 