By setting up two Pendulum cards in your “scale,” you could summon multiple monsters at once depending on their Levels.
Namely, you need a specific Ritual card, the Ritual monster it summons, and the required amount of monsters to fulfill the summon.
Introduced with Yu-Gi-Oh 5D’s, Synchro summoning takes advantage of the exact Levels (number of stars) of monsters, and introduces cards with white backgrounds.