Dynamic tension is almost like isometrics, except with an isometric hold you don't move the limbs and thus only hit the muscle at one angle. This is OK because muscles get stronger such that they have more strength 30 degrees each way from where you place them under stress.

The simplest way to demonstrate DT for yourself is to straighten your arms, cup your fingers together and pull the arms/shoulders outwards; then, move your straightened arms in an arc from waist to above your head. You should feel it in your shoulders and back.