Moeris, Attic Words, p.193.4-7 (Bekker)

βαλβῖδες αἱ ἐπὶ τῶν ἀφέσεων βάσεις ἐγκεχαραγμέναι, σἷς ἐπέβαινον οἱ δρομεῖς ἵν’ ἐξ ἴσου ἵσταιντο. διὸ καὶ οἱ κήρυκες ἐπὶ τῶν τρεχόντων “βαλβῖδι ποδὸς θέτε πόδα παρὰ πόδα” καὶ νῦν ἔτι λέγουσιν. Ἀττικοί, ὕσπληξ δὲ κοινόν.

‘Balbides’ are the grooved stones at the starting-lines, onto which the runners stepped so that they would have a fair start. For this reason the heralds, even to this day, say in the presence of the runners “On the ‘balbis’ for your foot put foot by foot!” This is the Attic usage; that of the common dialect is ‘hysplex’.

i.e. like the starting-line at Olympia.