Why does pragma unroll 1 mean not unrolling at all?

Question

If I have this code:

#pragma unroll 1
for (i=0;i<5;i++)
{
   a[i]=i;
}

I read somewhere said #pragma unroll 1 will prevent the compiler from unrolling the for loop. Why is that? And why does it not unroll the loop the first time, and run the for loop on the rest (ie a[0]=0; then run for (i=1;i<5;i++))?