Every time a row of four pieces of the same colours occurs on the board, this row is automatically removed from the board by the program.
When more then 1 row of the same colour is on the board at the same time, the program will ask the corresponding player what row to remove first. The row you can remove at that moment will be made clear with red crosses on top of the pieces.
Something similar occurs when you have the chance to remove a gipf-piece from ove a gipf-piece from the board. The piece that can be removed will get a red cross and the program will ask you if you want to remove it or not.
Even when you can remove a gipf-piece from your opponent from the board will the program ask you if you want to do this. In some situations it can be benificial to leave your opponents gipf on the board.