Bukkit

I have a question about villagers and carts. When we push a villager into a cart and then try to push the cart away with the villager in it, the villager jumps out.

I found this in the DefaultTrainProperties.yml file but there are no notes explaining what the values can be or what they do. Current settings of DEFAULT does not explain what they do, really. Does this mean "default" as in regular Minecraft behavior, or does it mean some sort of regular Traincarts behavior? What else could be the value besides Push and Link?
'''collision:
mobs: DEFAULT
players: DEFAULT
misc: PUSH
train: LINK'''

Please provide any additional information below.