What is the principle of iOS6’s Mail inside pull down to refresh animation effect

  ios, question

It was found that iOS6’s mail also joined pull down to refresh’s effect. A signal to pull down can be released, which is the effect of breaking off something like a drop of water to a certain extent. This animation is non-linear. What principle did it follow?
I have done the linear animation effect of UIViewAnimation before. Don’t you know that this kind of nonlinearity can be realized similarly?
In addition, the effect of deleting photos of inside seems to be non-linear.

Turning github has a similar drop-down deformation demohttps://github.com/dbsGen/SlimeRefres …