What is this `(*(T*)nullptr)` old trick?

I have just read Is declval<T>() the same as (*(T*)nullptr)? and just realized that I haven't heard of the (*(T*)nullptr) trick before. If I analyze that snippet, it doesn't give me too much information, beside that (quoting the answer)

you can get an instance of T in a decltype without needing to worry about T's constructor

and also the example found there is just very trivial:

struct B {
    A a;
};
typedef decltype((*(B*)nullptr).a) T1;

so, can please someone explain to me what this (*(T*)nullptr) trick is and why (and how) would I want to use it (beside of the example above)?