Naam abhyaas in surthee is basically chanting or reciting Naam in concentration. When chanting Naam one can do it using this body or purely in concentration. When Japping Naam purely in surthee, without the aid of the body i.e. tongue, throat, lips or any other part of the body, it makes the Surthee strong. Some paath of Gurbani should be done this way everyday. Sardar Raghbir Singh Bir has written that with abhyaas, the surthee becomes so loud and strong that when japping Naam in surthee, one cannot hear outside sounds. He gives example of travelling in a noisy train and not hearing anything when the surthee is involved in Naam Abhyaas.
Hirda is a place few inches above the navel. According to Gurbani this is the place where the mind should reside when it reaches utmost concentration:
ਇਹੁ ਮਨੁ ਨਿਹਚਲੁ ਹਿਰਦੈ ਵਸੀਅਲੇ ਗੁਰਮੁਖਿ ਮੂਲੁ ਪਛਾਣਿ ਰਹੈ ॥(This mind is stable when it resides in the Hirda and this is when the Gurmukh realizes his roots.)
The mind resides in the Hirda after extensive Naam Abhyaas.
The above is based on Bhai Sahib's writings. I suggest that Bhai Sahib's book Anhad Shabad Dasam Duaar and Charan Kamal ki Mauj for more details or engage in extensive Naam Abhyaas.
Kulbir Singh
ਸਤਿਗੁਰਬਚਨਕਮਾਵਣੇਸਚਾਏਹੁਵੀਚਾਰੁ॥