In north part of India, I have observed, Hindus only eat meat of goat/chicken beheaded at once. They say it "jhatakA"; literal meaning "at once". Is this concept backed up by scriptures ? If not then what way is prescribed to behead animal before consuming it ?
Even if one beheads the animal in one go still it will be an act of just a killing and not a sacrifice.
If sacrifice is being performed as per scriptures then first the animal is to be bathed then it has to be worshipped duly thinking it to be Lord Shiva.
The sacrificial post (the yup), the weapon are to be worshipped too. Otherwise the act is nothing but what happens in slaughterhouses.
Lord Shiva warns in this regard:
HanyAn-mantrena chAnena tvabhi-mantrya pashum priye |
Gandha-pushpa-akshateih pujya chAnyathA narakam vrajet ||
O Beloved, the animal is to be energized with this mantra and then worshipped duly with scents, flowers and rice grains before the sacrifice is being performed. Doing otherwise one will have to visit hell.
KulArnava Tantram 5.50
The mantra being talked about is the following:
Shivotkrittamidam pindam-tastvam shivatAm gatam |
Tadvudhyasva pasho tvam hi mAshivas-tvam shivohasi hi ||
Your body will be cut by Shiva; hence you will attain Shiva-hood. O Pashu, know this -- You are not a-Shiva, you are also Shiva.
This is a Bali mantra and the sacrifice is to be performed while chanting it.
So, if these or such instructions are not duly followed then it is just an act of killing an innocent animal. It does no good to anybody and it is not a sacrifice.
The meat is also not sacrificial meat (because it is not a sacrifice and more so because it is not offered to any deity).
In Hinduism, eating meat is not completely disallowed but the recommended method is to offer the meat to deities before eating. Eating sacrificial meat is completely allowed and it has no sins attached to it.
In this regard, see what Lord Shiva says:
Pitridevatayajneshu VaidahimsA Vidhiyate |
AtmArtham PrAninAm HimsA kadAchinnoditA Priye ||
In Yajnas meant for Devas and Pitrus (ancestors) sacrificial killing is allowed. But other than that the Shastras never allow prani himsa (killing) for one's own pleasure.
KulArnava Tantram, Chapter 5, Verse 45.
Even if the animal is beheaded in one go it's still not a sacrifice it's just an act of mere killing.