The main character finally gets a turn! Shizuno's ability may seem powerful in theory, but in practice it's unfortunately uncommon to be able to wield it effectively since it activates so late in the hand. It does, however, have a few nifty effects that you can take advantage of when playing against certain overpowered characters.
Here's the deal: Shizuno's ability activates each hand at the beginning of her 13th (or so) turn, when there are usually 12 tiles in her discard pile. When it activates, she gains control over the wall, causing the quality of her opponents' draws to gradually decrease. It's difficult for it to make that much of a difference since everyone has so few draws left by that point, but it can certainly be a frustrating experience for any opponents of hers that still haven't achieved tenpai.
The other effect of Shizuno's ability is that while it's active, it can prevent certain effects of certain other characters' abilities from occurring. This includes Saki's ability to draw a winning tile from the dead wall, Koromo's ability to draw a winning tile from the end of the wall, and Awai's ability to create a quad of kan ura dora, though there may be others as well. Keep in mind, Shizuno can't prevent these characters' abilities from activating entirely, but she can prevent their most powerful key effects from taking place.
So why can Shizuno control the wall like this? Well, once again, the origins of her ability are firmly rooted in anime logic. For a long time, Shizuno has loved taking long walks through the mountains around her hometown. She's explored them so often and so thoroughly that she's gained quite the affinity for the element of mountain, as it were. Now recall the Japanese terms for what we usually call the wall and the dead wall in English: mountain, and mountain peak. Indeed, her affinity for the dead wall is so strong that she can even wrest control of it away from such monsters as Saki and Awai.
I just love the bits in the anime when Saki and Shizuno casually walk past each other without realizing it. One girl gets intimidated by the creepy, intense aura of mahjong prowess radiating off the other girl until the camera cuts away from her perspective to show the other as the harmless, bubbly cutie we all know her to be off the table.
