If a person suffers from a reduced number of thrombocytes (=thrombocytopenia), he will have difficulty with hemostasis. In the case of a wound, the first step (spasms) and the third step (clotting) are still functioning, but there will be less or no plug formation.
These patients typically show small purple bleedings in their skin, called petechiae.
This disease illustrates an important point. In daily life, because of wear and tear, we suffer constantly from (small) bleeding. The vast majority of these bleedings are not noticed because they are very small and are resolved before becoming visible.
If a person however does not have enough platelets then plug formation is impaired. So, in that case, the spasms still occur (reducing the blood flow) and the third step (clotting) will also occur (because there is enough damage in that neighbourhood) but there will be a small amount of blood lost in the tissue. This becomes visible as small clots (purple or dark red). These petechiae most often occur where the tissue works most (wear and tear at joints, hand and feet) but can also occur in tissues inside the body, which are not visible.
The point this disease illustrates is that the hemostasis machine is working all the time to stop the bleeding in those (small) vessels that ruptured spontaneously all the time.