Incorperating rollerboard workouts in your training regiment is just one of many ways to improve your ski specific strenght. The muscle stimulation that occurs while working out on the board is pretty close to what you would encounter double poling on snow. The main difference being the fact that your abdominal muscles do not, as they would on snow, participate in the propulsion. It is exactly for that reason that it is recomended to include a good ab session following this workout.
The way I usualy proceed is the following. First you need to be warmed up and the rollerboard is not the best way to do this. I suggest going for a short running warm-up or do this session right after an easy workout. Once warmed up you get started on the roller board. It is actually hard to dictate a number of reps, a time or an angle at wich you should do this workout since different boards offer different resistances. One way I like to do things, is to start at a lower angle and do the maximum number of reps that I can at a moderate speed, then take a 30 to 40 second break during wich you set up your board at a higer angle. At this new incline repeat your max reps, but this time as fast as possible. Take a 2 minute break and then repeat both sets. I usually repeat this 4 or 5 times.
So however you decide to do your workout, Iím sure you can find a way to make your muscle scream !!!