The concept 2 indoor rowing machine, is one of the best pieces of gym equipment that you find in any good gym, or ideal to have at home.
The biggest problems that most people have when using the concept 2 rower, is that they have the side resistance far to high, which can cause excessive straining on your muscles, especially the lower back. Aim to have the resistance set no higher than 6 for men or 4 - 5 for women, unless your doing specific strength drills.
Other problems are going at an excessive stroke rate. When rowing on the concept, you should aim to use the return phase (bending of the legs) as a brief recovery - often called the recovery phase.
The simplest tip for the concept 2 rower, is to use your legs first, then your arms and upper body, then making sure that you return back with your arms first, bending the legs only once the handle has passed your knees. If the chain is going excessively up and down in a jerky movement, then your doing something wrong.