Well, human SOULs are extremely powerful! So powerful that they can persist even after death, thanks to their determination.

[This is a scientific fact, courtesy of himself and Undyne! True revolutionaries in the art of soul science!]

As for how... well, the barrier was constructed with a powerful magic equal to that of seven human SOULs. So, to break it, you need an equal power! Otherwise, anyone who comes through gets trapped, just like the rest of us. How tragic!