A TENS machine specifically designed for use during labour will have a BOOST BUTTON.
It may be integral to the TENS unit or it can be a hand-held device leading from the unit.
The Boost Button is pressed whenever you have a contraction to block the pain signals sent to the brain. By pressing it you switch the signal pulse rate you feel from a low setting (2 pulses a second) to a high setting (150 pulses a second). A continuous sensation is felt with the Boost Button on.
Between contractions the Boost is turned off to return to a low pulse rate. This helps your body to continue to produce endorphins.