It measures statistics like calories burnt and thrusts per minute.
This is the product description:
When the centre light is lit, the user is hitting peak performance and matching the rhythm set by the training programme. Too many lights and the user will need to slow down, and not enough lights will advise them to pick up the pace. A “+/-“speed button on either side of the gadget can also be used to adjust the speed of the training session ‘on the go’.”
One thing's for sure: I wouldn't share my sexual performance on Facebook.