Please look careful at this diagram. In the lower left corner black has played the triangled stone. White's stone can try to escape only by playing at A but black will easily capture it. Please figure it out by yourself! On the right side is a similar situation but black played at 1 and now white can escape after 2. Well, the point is: WHEN CHASING A STONE OR A GROUP OF STONES PUSH THEM INTO A NARROW AREA! The side of the board can be considered a narrow area as a stone on the first line has only 2 liberties.

  Another example of pushing the weak stones into a narrow area. After black 3 white's stones are captured.

  Here black went wrong as white has poked out his head. Please notice the big difference between this and the previous diagram.

  After black 1 white's marked stone can't be saved so black is connecting his own stones like this.

  Another big success for black! His marked stones are saved now.

FAILURE! Black's stones are separated and the marked ones will die.