If you jump and keep holding it then dash into / through something you'll fall slower, also if you hold jump when dashing down into a bouncepad you'll go a tiny bit higher.

However, if you were to dash of something then dash back and keep holding jump then the slow fall effect doesn't happen.

Can be useful in a couple areas. e.g bottomless pit to skip the moving platform and to slow fall to make dashing into the final dirt instantly a lot easier, instead of having to bonk on something first to slow you down.