From my readings from the EU, the Sith, or dark practitioners are always said to be using pain, hate or anger. And this is true to some degree, however it is overlooked that the Sith in particular have a code. The first line says what I want to stress.
Peace is a lie, there is only passion.
Passion can be any emotion, from anger, to hate, to love, to want, to your own ambitions and your own furthering of your goals.
Darth Bane, the Sith Lord who implemented the Rule of Two, drew inwardly on not only his own pain, or his own rage, but from a sort of inner fire, or drive to be better than others. To become the most powerful. You may see that it is expressed as anger to the others, or hatred for those individuals. Whoever you see that they are using their emotions in general.
Darth Vader, when he fought, focused the Force inward and fought as a cold, calculating machine. Using the Force to bolster his strength and focus. Revan was a conduit for both the dark and the light and had the energy flow within him. Even while imprisoned, he still focused on the aspects of the Force, and how it is an energy. Emotions are a source of energy that Sith can harness, whereas the Jedi focus on control over their emotions, because of the overwhelming nature that emotion brings to reason.
When a Sith meditates, they are focusing their emotions. Bringing them to a boil, gathering them to be able to unleash them, or harness them to enhance themselves. The Sith gather the Force just like a Jedi does. It is just a different aspect of the Force.
A major showing of this is the Force powers of Battle Meditation. Both Sith and Jedi have this skill, albeit it is rare to find amongst both ranks. This was also demonstrated with Grand Admiral Nial Declann. Meditation is not only for calming the mind, or emotions, but focusing it.