If I'm right, by dynamic tension you mean executing a waza very slowly while flexing the muscles intensely. The theory is that by doing the movements slowly you make sure your doing it with perfect form. The dynamic tension of the muscles speeds the development of muscular memory and allows you to perform the technique faster, stronger and more relaxed during normal practice.
This is not really an aikido excersize. I learned it from my uncle who used to study kung fu. I practiced it a lot while studying tae kwon do. I used it with my form development and punching/kicking drills. I have to admit it works very well for linear and crisp techniques. In tae kwon do my strikes where faster and more powerful. I've tried it with aikido with limited success. Aikido is more of a circular style and its movements are not geared towards the excersize. Although, I have had success with a modified form of dynamic tension.
I use more of a tai chi chuan methodology. I perform the waza very slowly using deep diaphramic breathing (Breathe into your hara). Making sure the breath is slow and continuos. I use one complete breath for each waza. My body is not so much tense, as I am concentrating INTENSELY on proper extension. This way I get to work on the proper principles of aikido (centering, extension and fluidity of motion) without having to worry about uke clocking me in the face.
I tend to keep my eyes in a semi-closed position and visualize my extension of energy. As I begin I breathe in, imagining myself absorbing all the surrounding energies into my hara. I imagine myself projecting all the energy I accumulated outwards at the end of the waza.
Peace and Blessings.