mutable XOR alias is also key here

For a typical type Goose it's fine if two threads can both look at the Goose (via a reference, pointer or whatever), so long as nobody can mutate the Goose. If thread A finds that the Goose is Happy, thread B weighs the Goose and finds it to be Heavy, and thread A again measures the length of the Goose as 860 millimetres this is all fine, it won't matter if (by the vagaries of hardware) the weight is measured before the length or after, there's no difference.

In Rust this is reflected in &Goose, the immutable reference to a Goose, being Send, ie a thing you can give to other threads. The mutable reference &mut Goose is not Send.