Vali was a very powerful warrior and possessed many boons against defeat and failure. Sugriva doubted if Rama, was a match for Vali.
Rama showed his prowess by performing wondrous feats. Sugriva was not satisfied. He pointed out a sal tree and asked Rama if he could pierce it through with an arrow:
Rama let off an arrow from his bow, which shot through not one but seven sal trees in a row. After touching the ground beyond the seventh tree the arrow returned to its quiver.
Sugriva was aghast and knew that victory was on his side.