To be able to address the slit with a knife, you'd need to be able to move your hand in a fourth spacial direction, that of the liminal barrier. The only part of you able to do that is your soul, which exists partially in the Aethyric realm and is able to delve deeper into it, most commonly during dreams. When magic enters a soul, it gets closer to the realm that birthed it, which is what makes magic so powerful and dangerous. Which means that if you pass magic back and forth between your soul and your body at the edge of the slit, sawing back and forth like a cheesewire, then...
With an indescribably unpleasant noise that echoes off the mountains, the tear grows unevenly and jerkily in the direction of your effort of will, widening the hole between reality and the liminal realm. As you circle around it you wince at the uneven shape of it compared to the slit that it had been, but at least it works. It's slightly alarming to create a rent in reality with such a small effort, though it makes sense that it's so much easier to enlarge an existing hole than to tear a new one.