On what principle does S - conversion to DES work?

Question

Cryptography

After modulo addition with the keys, we get 8 consecutive 6-bit blocks B . After they are converted to 4-bit using S - transformations. On what basis does this happen?

Answer 1

https://ru.wikipedia.org/wiki/DES#.D0.A1.D1.85.D0....
S-transformation is just a substitution from tables (S-boxes).
The first and last bits form the row index in the S-box.
The four bits in the middle form the column index in the S-box.
Each B has its own S-box.