The original quote is a bit unclear, but it does refer to being on a playground:
"Recall the pure joy of riding on a backyard swing: an easy cycle of motion, the momentum coming from the swing itself. The swing carries us; we do not force it. We pump our legs to drive our arc higher, but gravity does most of the work. We are not so much swinging as being swung."
I found it excerpted in Getting Things Done, where the author presented it as the ideal for how one should approach their life, both professionally and personally.
"Recall the pure joy of riding on a backyard swing: an easy cycle of motion, the momentum coming from the swing itself. The swing carries us; we do not force it. We pump our legs to drive our arc higher, but gravity does most of the work. We are not so much swinging as being swung."