(See also the one-sided mode, for when you only control one side's moves).
This is a tool to encode/decode data in chess games. It first encodes the input data as a bignum, and then encodes the bignum in the move choices in the chess game. The "without blunders" mode uses p4wn to try to avoid playing bad moves. This mode is less likely to arouse suspicion among actual chess players, but results in longer games.
See my blog post for more information.