The Ambassador Seeds are described as thus via in-game text:
Ambassador Seeds were first documented about 150 Earth years ago. They occur about once every 400 days if the trees remain healthy. Natural seed swaps occur between pairs of seeds that (we now know) dropped simultaneously from healthy trees in paired sphere. When each seed was touched by species in sphere the swap occurred - sending an "ambassador" from each sphere to the paired sphere.
Location of the swap is defined by the locations of the pairs of seeds. After the first swap the seeds recharged quickly - allowing for a quick return. "First meetings" were intense but naturally short (it was quite a surprise for both the Mofang and us.) Over time the seeds required more time to recharge - producing longer visits between species.