It would appear that the spell is an incantation (e.g. When you say it, it casts the spell). When it's cast, it simply follows the line of the wand rather than requiring any specific wand movement.
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
and
‘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
and
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