Su Er had long since grown used to Snape's habitual barbs. He gathered his thoughts and looked earnestly at his dear teacher.
He had to listen carefully to the details of the lesson. Unlike many spells taught in class, this spell had to be learned with an experienced adult wizard present, and in an open space.
Otherwise...
Splinching was one of the milder outcomes. With magic, a little pain was all it took to put things back.
The most terrifying outcome was being Splinched with part of the body stuck inside a wall or some other inexplicable place.
Imagine a length of intestine stuck inside a wall or a rock...
"First, to be precise, Apparition is a combination of two spells: Disapparition and Apparition."
"Put your wand away. You don't need it to learn this."
"During Apparition, the most important thing is to follow the three Ds: destination, determination, and deliberation."
"When moving, you need a clear, definite impression of your destination. Now, look at the white circle in front of you and form a clear impression of it in your mind. I want you to put aside all the clutter in your head and focus solely on that."
Su Er nodded and turned his gaze toward the white circle a short distance away. This step was very difficult for most people, because they struggled to keep only what they saw in their minds.
Perhaps they could manage it for ten or twenty seconds, but an active brain would not settle for being quiet. All sorts of thoughts would interfere, such as what might happen if they were Splinched, or got stuck in the ground...
But this was no problem for Su Er. Having learned Occlumency, he could accomplish it very easily.
A minute later, Snape gave a slight dip