[nneonneo]

Essentially correct. 33 sets up the pointer to appear to glibc as if it were a small, “normal” 32-byte allocation. The default behavior for glibc free() with a small allocation size will be to put it into the thread cache (tcache), which makes it immediately available to be reused by the same thread.

Although you should obviously never write code like this by yourself, understanding weird details like this is very helpful when exploiting memory error bugs in software, as it lets you understand how precisely to subvert the memory allocator to give you access to desired parts of memory.