What is Kriya Yoga? It comprises three parts. Patanjali says tapaha.
1. Tapas is endurance, acceptance.
Say you are traveling in a plane; it is a long-distance flight. You are sitting and your legs are getting numb. You are tired but you keep sitting. You feel heavy but still you keep sitting. You cannot say, "Oh! I cannot sit now. I am going to get out of the plane". No way. If the plane is delayed, held up in the air, you have to be there. There is no choice. Now, if you had a choice, then you would not sit for eight hours like that in one place. No way. But in a plane, you sit, accepting it willingly, without grumbling. That is tapaha.
2. Swadhyaya - is being alert, self-study, observing the thoughts, emotions and where the emotions come from. Where do the thoughts come from? What is happening inside you? Studying yourself is swadhyaya.
3. Eshwarapranidhana - means devotion to the Divine, love for the Divine. These three things make up Kriya Yoga. What does it do? What do they all do to you?
Kleshatanukaranarthascha- they reduce the suffering. They reduce the misery in life. Samadhi bhavanarthaha- gives rise to samadhi, equanimity.
link: Patanjali Yogasutras
That is what it is really called, Tapas, we have no choice anymore.
We said 'yes' to it and went on that flight and now we sit here and have to go by it, that means Tapas.