Patanjali continues to detail the benefits of practicing the yams and niyamas to motivate the practitioner, here he talks of the benefit of niyama of ‘tapas’,
kaayendriya siddhirashuddhi kshayaat tapasah
Kaaya : body
Indriya : senses
Siddhi : perfection / supernormal powers
Ashuddhi : impurities
Kshayaat: from the removal of
Tapah: the niyam of ‘tapa’ or austerity
Practice of tapa leads to removal of impurities of body and senses and enables one to perfect them/attain supernatural feats.
When you heat water, impurities are left behind and it changes to a subtler form — that of vapour. Similarly when a being gives himself pain and generates the heat of tapa, the body and senses get purified and he/she becomes entitled for subtler / higher dimensions.