Danda means a long rod, and vat means like. Before a superior, one has to fall down on the ground just like a stick, and this sort of offering of respect is called dandavat. Atri Rsi offered his respect to the three deities in that way. They were identified by their different carriers and different symbolic representations. In that connection it is stated here that Lord Visnu was sitting on Garuda, a big aquiline bird, and was carrying in His hand a disc, Brahma was sitting on a swan and had in his hand kusa grass, and Lord Siva was sitting on a bull and carrying in his hand a small drum called a damaru. Atri Rsi recognized them by their symbolic representations and different carriers, and thus he offered them prayers and respects.