S tar Spring

Star Spring is a Colorless battlefield (no errata): "The first time a player plays a non-token unit here each turn, they may move another unit they control here to its base."

Three things players most often get wrong: the trigger is per-player (your opponent playing a unit there doesn't use up your trigger), it only fires on a play — moving a unit in doesn't count — and the ability goes on the chain like any triggered ability, so it can be reacted to. It also moves another unit, so it can't bounce the unit that just triggered it, and moving a unit to base doesn't cost you control of the battlefield as long as you still have a unit there.

What's the board state you're looking at? I can give you the exact sequence.