John Gill Commentary 1 Samuel 20:36

"And he said unto his lad, Run, find now the arrows which I shoot. And as the lad ran, he shot an arrow beyond him." — 1 Samuel 20:36 (ASV)

And he said unto his lad, run, find out now the arrows which
I shoot
He no doubt told him the mark which he should shoot at, the stone Ezel, and bid him look out about that for them:

[and] as the lad ran ;
before he had got to the mark:

he shot an arrow beyond him :
or it; beyond the lad, or beyond the mark he shot at; purposely shooting with great strength, that he might exceed, and thereby give notice to David how things stood, which was the sign agreed on.