Yoga is indeed all about the mind, and there are quite many yoga enthusiasts strongly believing in the power of thoughts for their own particular reasons.
There are people believing in, "Things will manifest as you think and desire. If you gather the power of thoughts to be focusing on something that you desire, and strongly believing that all your desires will come true, and it will come true as you desire." That's their freedom of thinking and desire. However, it's not part of the yoga practice.
The essence of yoga is about the annihilation of the modification of the
mind. It's about silencing the thought activities. It's about the annihilation of desire, or the state of desirelessness. It's about realizing selflessness through the annihilation of ignorance and egoism, of attachment, identification, desire of craving and aversion, comparison, judgment and expectation. It's about realizing non-separateness or oneness, transcending duality, of good and bad, positive and negative, meaningfulness and meaninglessness, success and failure, happiness and unhappiness, pleasurable enjoyment and suffering. It's about respecting the law of cause and effect, and the truth of selflessness and impermanence, which also leads to respecting the perceived
reality as it is, not necessarily pleasant, desirable or agreeable, and it's changing at its own cause and pace. Free from the egoistic desire and intention to be controlling or changing the perceived reality to be or not to be in certain way.
Somehow, yoga practice has been being manipulated into a tool for developing mind power to be gratifying all kinds of desires.
Here is a simple inquiry towards the power of thoughts.
By focusing and thinking very hard and strongly believing in the earth is square, doesn't make the earth become square.
By focusing and thinking very hard and strongly believing in those who had passed away will come back alive, doesn't bring back the dead to be alive.
By focusing and thinking very hard and strongly believing in the world will be free from all kinds of ignorance and suffering, doesn't make all kinds of ignorance and suffering disappear from the world.
By focusing and thinking very hard and strongly believing in "(I) am the champion of any competition who will win all competitions even without the need of any effort of training and preparation," doesn't make (I) become the champion of all competitions.
By focusing and thinking very hard and strongly believing in "This apple will become an orange," doesn't make the apple turn into an orange.
Regardless of how positive and optimistic is the thinking mind, it doesn't change the truth of selflessness and impermanence in all kinds of names and forms being what they are, as they are, and constantly changing, arising and passing away, governed by the law of cause and effect.
What everyone want to think and believe is their freedom, however, for the sincere yoga practitioners, they must free the mind from ignorance by inquiring towards the truth of everything.
One can be appreciating good condition or good health when it's available, but without attachment towards good condition or good health. One can do one's best to maintain good condition or good health and avoid sickness or injury, but there's neither craving towards sickness/injury-free nor aversion towards sickness and injury. If there is sickness or injury arise in this body, then either it can do its best to take rest, allowing the system to heal itself, or it can receive certain medical treatment if necessary, if available, and allowing the medical treatment to do whatever it needs to do, while respecting the body will be recovering at its own cause and pace, or not. There's no fear or aversion towards selfless impermanent changes of decay, old age, weakness, sickness, injury and death, or separation from the world of family and friends and things that one loves and appreciates. There's no need the power of thoughts to be desiring this or that, and to be controlling or changing the perceived reality to be or not to be in certain way.
The power of thoughts is more about from the rise of thoughts/intentions/desires that are whether under the influence of ignorance, egoism and impurities, or not, that influencing the minds generating actions/reactions/expressions that could be beneficial or harmful to oneself and/or others and the surrounding environment.