I am relatively new to advanced Sudoku solving techniques and I would be grateful if someone could explain the logic behind the following.

I have a puzzle with a series of 6s in it as follows:

R1 @ C1, C2, C6, C8 & C9

R2 @ C2, C6, C7 & C8

R3 @ C2, C6, C7 & C8

R4 - 6 as solution in C5

R5 @ C7 & C8

R6 - 6 as solution in C3

R7 @ C4, C7 & C8

R8 @ C2 & C4

R9 @ C1, C4, C8 & C9

The matrix formed by the 49 squares inside the outermost columns and rows contains between 2 and 5 6s in each row or column (except where 6 has been found as the solution in a row or column).

The 6s lying outside the inner matrix, but sharing a column with it i.e. in R1C2, R1C6, R1C8, R9C4 & R9C8 are deemed not to be candidates and can be removed, but the 6s not seen by any of the rows or columns of the inner matrix i.e. in R1C1, R1C9, R9C1 & R9C9 remain.

Help!

Kevin