The difference between Hot Yoga and Hatha Yoga
We know that when it is warm, our muscles are supple and we can flex and bend and do yoga asanas that we cannot easily do when it is cold. Hot yoga practices like Bikram yoga heat the room to between 100 and 105 degrees Fahrenheit to make it easy to twist and bend to do various asanas.
Alternately, one can generate internal heat using the Ujjayi breath and channel this heat through the body using the bandhas. The body is, then, said to become malleable due to this generated heat (Ref: Flexibility vs Malleability in asana practice).
Outwardly, the effect can be the same in the accomplishment of the final pose whether the heat be provided externally or generated internally using the Ujjayi breath. However, there is an huge difference in terms of the ensuing mind states when using the bandhas and the Ujjayi breath to generate heat than when the heat is supplied externally. In the former case, the states of mind reached like joy, effulgence, outpouring of love etc. are unparalleled. This is because, in this case, one operates at the level of the Prana.
Nevertheless as most Hatha Yoga practitioners know, the states of mind reached vary from one day to another. For Prana to flow well, the obstructions to its flow need to be minimized (Ref. Prowess in Poses). Adequate rest, light diet, empty stomach and a clean colon go a long way in making the Prana flow well in one's body.
The sleeping Kundalini is awakened by this heat and it hisses and straightens like a snake struck by a stick (Hatha Yoga Pradipika 3.68). SwatmaRama, the author of Hatha Yoga Pradipika, says, that for the reasons mentioned above, one must practice Moola bandha everyday.