فَقُلْنَا اضْرِبُوهُ بِبَعْضِهَا
So We said;"Strike him (the dead man) with a piece of it (the cow)."
meaning, "any part of the cow will produce the miracle (if they struck the dead man with it)."
We were not told which part of the cow they used, as this matter does not benefit us either in matters of life or religion. Otherwise, Allah would have made it clear for us. Instead, Allah made this matter vague, so this is why we should leave it vague.
Allah's statement,
كَذَلِكَ يُحْيِي اللّهُ الْمَوْتَى
Thus Allah brings the dead to life,
means, "They struck him with it, and he came back to life."
This Ayah demonstrates Allah's ability in bringing the dead back to life. Allah made this incident proof against the Jews that the Resurrection shall occur, and ended their disputing and stubbornness over the dead person.
Allah mentioned His bringing the dead back to life in five instances in Surah Al-Baqarah.
First Allah said,
ثُمَّ بَعَثْنَاكُم مِّن بَعْدِ مَوْتِكُمْ
Then We raised you up after your death. (2;56)
He then mentioned the story about the cow.
Allah also mentioned the story of those who escaped death in their land, while they were numbering in the thousands.
He also mentioned the story of the Prophet who passed by a village that was destroyed, the story of Abraham and the four birds, and the land that comes back to life after it has died.
All these incidents and stories alert us to the fact that bodies shall again become whole, after they were rotten.
The proof of Resurrection is also reiterated in Allah's statement,
وَءَايَةٌ لَّهُمُ الاٌّرْضُ الْمَيْتَةُ أَحْيَيْنَـهَا وَأَخْرَجْنَا مِنْهَا حَبّاً فَمِنْهُ يَأْكُلُونَ
وَجَعَلْنَا فِيهَا جَنَّـتٍ مِّن نَّخِيلٍ وَأَعْنَـبٍ وَفَجَّرْنَا فِيهَا مِنَ الْعُيُونِ
لِيَأْكُلُواْ مِن ثَمَرِهِ وَمَا عَمِلَتْهُ أَيْدِيهِمْ أَفَلَ يَشْكُرُونَ
And a sign for them is the dead land. We give it life, and We bring forth from it grains, so that they eat thereof. And We have made therein gardens of date palms and grapes, and We have caused springs of water to gush forth therein. So that they may eat of the fruit thereof ـ and their hands made it not. Will they not then give thanks! (36;33-35)
وَيُرِيكُمْ ايَاتِهِ لَعَلَّكُمْ تَعْقِلُونَ
and shows you His Ayat (proofs, evidences, etc.) so that you may understand