Cardcaptor Sakura is using one her Clow Cards, The Float, to see how high she can go without needing more equipment. According to its anime appearance, The Float appears to be a balloon whose magic makes things weightless, but it doesn't grant mid-air maneuverability either way. That explains how Sakura can stay balanced on top of a rising balloon - she's independently weightless.
Cardcaptor Sakura is using one her Clow Cards, The Float, to see how high she can go without needing more equipment. According to its anime appearance, The Float appears to be a balloon whose magic makes things weightless, but it doesn't grant mid-air maneuverability either way. That explains how Sakura can stay balanced on top of a rising balloon - she's independently weightless.