This can come in different forms, such as:
1. The heroine wears nothing on her face at all, while male heros wear masks
2. The heroine wears a significantly less concealing mask, like a domino mask that leaves hair eyes and facial structure exposed.
3. The heroine wears a mask/helmet, but it is much more frequently removed than male counterparts, who may never have theirs removed.
The trope comes into play because the male viewing demographic is willing to suspend its disbelief to see the woman's hotness. Similar to Paper-thin disguise or Wig Dress Accent, but in this case it is literally no disguise at all. See Clark Kenting.