If you are teleported by a command block with the /tp command and relative coordinates while you are in the air (Either from jumping or flying, possibly falling too) and moving quickly, the relative teleport will succeed but after about half a second you will rubber band back to your arrival location. For example, I have a command block clock of /setblock ~ ~1 ~ minecraft:redstone_block 0 destroy hooked up to a /tp @a[x=0,y=0,z=0,dx=1,dy=1,dz=1] ~1 ~1 ~1. Flying through the region or sprint jumping through with Speed II will successfully teleport me from, say, 0,0,0 to 1,1,1, but after a moment (Of continuous movement) I will return to 1,1,1. This does not happen while sprint jumping or while sprinting with Speed II. Only with both or when flying.