Also, Rush is the first keyword not limited to spirits/braves. There are also magic cards with rush.
Rush encourages mixing colors, as it requires keeping a different color symbol on your field to activate, designated on the card. Following the card's first effect, as long as you have the conditions fulfilled, the rush effect activates after. The actual effect of rush varies from spirit to spirit.
Starting from BS24, symbol conditions have also included 2 of the targeted symbol.
[LV1][LV2] (When Summoned) Return 1 opposing spirit with 2000BP or less back to your opponent's hand.
[Rush(Condition:Green Symbol)] While you have a green symbol, the effect below remains active.
Green: Move 1 core from the void to this spirit.
This means after you resolve the "When Summoned" effect of this spirit, you can activate its "Rush" effect if you fulfill the rush condition.