One of the most useful exercises for runners, the Bent Knee Calf Raise hones in specifically on the Soleus muscle in the calf. This is the muscle that takes a huge amount of load when running.
Like a typical Calf Raise you will be rising up and down on the toes of one leg at a time. The difference with this exercise is that your working leg will remain bent throughout. As you lower your heel below the toes your knee should be bent and it should remain bent all the way back up to the top of the movement.
You will find that this is a much more challenging exercise than the typical calf raise. As a result I would recommend you start with bodyweight while you perfect the technique, before adding a weight if you feel comfortable.