Running up hill is hard, doing it down hill is a good workout for leg turnover. This type of training can be used before hard intervals on the track or the roads. This will increase your speed as this type of training triggers off more muscle fibres, and makes the neural pathways more eficient. Running down hills is taxing on the quads, as the muscle fibres lengthen under tension, this is called eccentric loadind, its the very opposite to the action of running up hils which is a concentric muscle action.
I train athletes in Cyprus who do this type of training on the "Train smarter not HARDER" coaching method. A recovery day will be needed after this type of training.
Try 5 reps down hill, staying relaxed. Recovery will be a walk up, or fast walk up. Make sure the hill is around 200m long.As soon as you are at the top go again.
Allan