Hearing / Seeing etc is used with the meaning as  ‘As somebody heard / saw ‘ OR  ‘When somebody heard / saw ‘.  We can combine two sentences using this structure. It is to be specially noted that the subjects of both the sentences to be combined will be the same in this structure.

• He felt the gas leaking.  At once he turned off the cylinder valve.
• Feeling the gas leaking, he turned off the cylinder valve.(combined)
• The thief saw the police jeep coming.  Suddenly he hid behind a tree.
• Seeing the police jeep coming, the thief hid behind a tree. (combined)
• The child saw a snake.  Immediately it began to cry.
• Seeing a snake, the child began to cry.(combined)
• The boys heard the school bell.  At once they ran to their classrooms.
• Hearing the school bell, the boys ran to their classrooms.