Timing mistakes in freestyle

The usual problems in this area are: the movements of one arm get out of synch with the other, body rotation gets out of synch with stroking movements, the downsweep is started too soon, the arm in front glides too long before starting the downsweep, and poor synchronization between the leg and arms.

• The recovering arm should enter the water when the stroking arm is starting the insweep. There are two possible mistakes in this respect. The recovering arm can enter the water either too early or too late in the stroke cycle. If swimmers allow one arm to enter the water during the downsweep of the other, they will be rolling in the wrong direction and this will create pushing drag for the recovering arm. To correct this motion, they will have to slide the recovering arm into the water and outward rather than forward until the insweep of the other arm begins and they can start rolling in the opposite direction. If swimmers put the recovering arm too late, that is, after the upsweep has begun, their insweep will generally be wide and ineffective. Swimmers must roll toward the stroking arm during the insweep if they want to use it to full advantage. If the body is rotated in the opposite direction, they will tend to slide the arm under the body or they will reduce both the length and the inward movement of the arm and push straigth back, outside the confines of the body, in one long upsweep. 
• Because the body should always rotate in the same direction the arm is moving, problems with both rotation and arm synchronization usually go hand in hand. The entire rigth side of the body should rotate down when the right arm is traveling down and up. The same is true for the left side of the body during the left armstroke. Rolling in this manner is very natural. The arms tend to pull the swimmer's freely suspended body in the same direction they are moving. When their armstrokes are syncronized incorrectly, swimmers will usually shorten or eliminate a particular phase with one armstroke to rotate the body in the proper direction for the other armstroke. For some swimmers, however, particularly those who have been taught to swim with a flat body position, it is not uncommon to resist the tendency to rotate altogether. In this case, they inhibit the natural tendency to rotate in synch with the lateral and vertical movements of the arms and the body torques out to the side, increasing form and pushing drag. Some swimmers also compromise the propulsive aspects of their armstrokes because they reduce the lateral and vertical movements of the limbs too much to prevent rolling.
• The third problem is very common because it is natural for swimmers to commence pushing against the water as soon as the hand enters the water. There are two very important reasons that they should not do this. First, they will be completing most of the propulsive phase of the other armstroke after the arm in front has entered the water. Consequently, before they start sweeping the entering arm downward, they need to keep it streamlined and in line with the body until those propulsive movements have been completed. Second, they may tend to terminate the upsweep of the stroking arm prematurely if they start pressing down and back with the other arm immediately after it enters the water.
• Swimmers who glide too long make just the opposite error. They do not start sweeping down with the front arm until the other arm has completed its recovery and has entered the water. This mistake is commonly referred to as a catch-up stroke. This term is used because the downsweep of one arm is delayed until the other arm is almost extended beside it. In a sense, therefore, the entering arm is catching up with the one in front. The reason the catch-up  stroke is a mistake is really very simple. I f swimmers are not applying propulsive force, their forward velocity will be decelerating. Any action that unnecessarily prolongs the interval of time between the end of the propulsive phase of one armstroke and the beginning of propulsion with the other will, therefore, reduce average velocity per stroke.
• Poor timing between the arms and legs is seldom a problem for swimmers. Swimmers seem intuitively able to coordinate the leg beats and armstrokes regardless of the rythm they use. Some swimmers, however, tend to deemphasize or overemphasize certain leg beats within a stroke cycle, even when the timing of those beats is correct relative to the armstroke. These minor and major leg beats probably indicate a problem with a particular phase of the armstroke they are paired with. For example, a swimmer who uses a small insweep will tend to deemphasize the accompanying kick. Likewise, when the kick accompanying the upsweep is minor, that phase of the armstroke has probably also been deemphasized. 

Swimming Fastest (by E.W. Maglischo)