I've read the OG manga and still don't understand the hollow stance even with this chapter's explanation.
And why does Hyunckle look like Gorilla Gachi in ikemen mode?
tldr, after unleashing a very powerful attack, there will be a short amount of time you will be showing an opening, this applies even to Vearn after unleashing his Tenchimatou. However, against strong enemy like Vearn or Hadlar, that amount of time is so small that unless you are willing to take a hit and then strike back immediately like avan did here, or Popp reflect Vearn spell back with the Mirror of Shahal, it is not possible to take advantage of. That's why this method of attacking is applied in both cases