Kriya Yoga through Shri Patanjali
According to Shri Patanjali, Kriya Yog is the practice of Tapa(Penance, Swadhyay(Self-Study) and Ishwar Pranidhan(Divert towards the God) together.
Tapa
Definition of Tapa is, living and doing act against the physical and sensual comfort. Now here is the question that what is this, and how it can be possible?
Actually, in the normal daily life we are the not honest to itself. For example, we think, nice and thick bed is give nice comfort but next day we get back pain. To eat spicy food we becomes very happy and feel good but this feeling of goodness is real? No, it is brings lots of problem, physically and mentaly both and these all obstacles (attachment towards sensual comfort and non stable and quick mental enjoy) stop us to find the settlement of the mind, misunderstand the reality and looses our value and make unpure ego.
"To attract and attach with the sensual subject but non stable benifit, is the actually is the beginning of our problems and sorrow"...Yogacharya Kashinath
When we understand the fundamental principles of nature(tattva gyan) then we find attraction towards comfort is actually beginning of the pain and then we tried to sleep on the normal bed, eat the normal food and think for surviving rather than enjoying then we improve quality and stamina of our body and mind too and use this human body to liberate form life cycle. We will find, for the small things our mind was irritating now due to practice it is getting settle, more stable and powerfull.
Tap means heat and this heating is take place in the mind, through body and mind itself and due to this energy we able to do some extra-ordinary things. In the Tap(a) section of Shri Patanjal ofcourse can also practice Asan(physical yogic exercises) and category and quantity of Asan can be depends on the practitioners quality.
Swadhyay-Study to itself:
Swadhyayadist devatasamprayogah I (44,2 Yogsutra)
Through self study comes the realisation of the intended deity.
(Self study is also a kind of strong mental purification, it works also to come down ego and improve the capacity of self realisation. When it establishe we have the capacity to see the truth and we becomes pure and soft and this is the power why we can introduce with intended deity...Kashinath)
Ishwarpranidhan-Surrender to God:
Samadhisidhirishwarpranidhanat I (45,2 Yogsutra)
By surrender life in God Samadhi becomes perfect.
These all above practices and theories are also Kriya yoga but these theories are from Patanjali Yogasutra.