Det verkar som att stavningen är en incantation (t ex när du säger det, kasta den stavningen). När den är gjuten följer den helt enkelt strängen på tappen istället för att kräva någon speciell trollrörelse.
Harry ignored her. He had just found an incantation (Sectumsempra!) scrawled in a margin above the intriguing words ‘For Enemies’, and was itching to try it out, but thought it best not to in front of Hermione. Instead, he surreptitiously folded down the corner of the page.
Harry Potter and the Half-Blood Prince
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‘SECTUMSEMPRA!’ bellowed Harry from the floor, waving his wand wildly.
Blood spurted from Malfoy’s face and chest as though he had been slashed with an invisible sword. He staggered backwards and collapsed on to the waterlogged floor with a great splash, his wand falling from his limp right hand.Harry Potter and the Half-Blood Prince
och
A few of them stumbled, one or two of them bound in ropes, but those climbing on to the rock behind them merely stepped over or on the fallen bodies. Still slashing at the air with his wand, Harry yelled, ‘Sectumsempra! SECTUMSEMPRA!’
Harry Potter and the Half-Blood Prince