The additional oxygen one gets while still breathing hard after finishing the exercise. While exercising, lactic acid forms up after the saved glycogen in your tissues are used up. It makes up in your tissues. It is what makes your tissues burn. Its development is what makes your tissues tire and give out.
That means that you can't in any way test or see or smell the service but rather you have to rely on the provider's promise that it will be so. That's why it is intangible.
