DX9 and UpdateSkinnedMesh

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Does anyone understand the math behind the DX9 function UpdateSkinnedMesh() ?
How are these new matrices applied to the vertex buffer, programmically?
Reason I'm asking, is I might try to do it myself if I can't get UpdateSkinnedMesh to work.


for (UINT i = 0; i < Bones; ++i)
D3DXMatrixMultiply(&m_boneMatrices[i],&pMesh->exBoneOffsets[i], pMesh->exFrameCombinedMatrixPointer[i]);

void *srcPtr;
void *destPtr;
pMesh->MeshData.pMesh->LockVertexBuffer(D3DLOCK_READONLY, (void**)&srcPtr);
pMesh->exSkinMesh->LockVertexBuffer(0, (void**)&destPtr);

pMesh->pSkinInfo->UpdateSkinnedMesh( m_boneMatrices, NULL, srcPtr, destPtr );

pMesh->exSkinMesh->UnlockVertexBuffer();
pMesh->MeshData.pMesh->UnlockVertexBuffer();


Couple questions on the above code:
1. If I have only one mesh, can the source and dest vertex buffers be the same?
2. If I set all the m_boneMatrices matrices to Identity and call UpdateSkinnedMesh,
will my vertex buffer change? (just as an experiment)

Because I tried both 1&2, and some of my vertices change, like get
zeroed out. Not sure what is happening. Matrices look ok.