The Burrows-Wheeler-Transform takes a string with length n, creates a matrix with n rows by shifting this string one position to the left for each row. Then the rows are sorted by the first column in lexicographical order. Then the last column will be submitted.
Why taking the last column? On Wikipedia there is an example with the string "^BANANA|". After sorting the first column is "AAABNN^|" and the last column is "BNN^AA|A". Using run length encoding it would be better to use the first column because of "AAA". So where are the advantages in taking the last column?