Final answer:
Platelet plug formation is the second mechanism of hemostasis, involving the clumping of platelets to form a temporary plug that seals the vessel opening and prevents further bleeding.
Step-by-step explanation:
Platelet plug formation is the second mechanism to be activated during the process of hemostasis. When a blood vessel is damaged, platelets clump together to form a temporary plug that seals the opening in the vessel. This plug helps to prevent further bleeding while the body repairs the vessel. Platelets release chemicals that contribute to the process of hemostasis, such as von Willebrand factor.