When making the transition from the baseline to the net, the back swing or racquet preparation should get shorter and more compact with each step to the net.  You are closer to the person hitting the ball and don’t have the have time for a full and formal swing. If the ball is going to hit the ground and is not to be taken as a volley, the emphasis should be made on the proper front contact point and full follow through. The ultimate example is a half volley. 

If you have made it close enough to make a volley, remember your first volley should be for placement and control and your second volley should be made at your net destination and designed to be more aggressive or to win the point.  The volley is said to be an abbreviated stroke with the preparation phase designed to present the racquet face to the ball and look at your target area.  In addition, there is less need to generate power and more need to control the power supplied by your opponent.