When you use /effect <name> minecraft:levitation <time> 1-127
the effect is as expected. But using levels 128-254 the effect is basically inverted pulling you down to the ground. This effect is strongest at level 128 and weakest at 254. Level 255 (max) is an exception though: you can't jump nor fall and just stay at the same altitude. Levitaition levels 128-255 also prevent you from picking up items or experience.