What does "teeth-rattling heap" mean?
Solution 1:
Because Hagarty was pushed over backwards, the fall was awkward and uncontrolled, leading to an ungainly posture on the ground. The force and shock of the fall made Hagarty's jaw open and close, with the teeth knocking against each other, causing a rattling sound.
The teeth-rattling heap condenses this into a short expressive phrase.