Any time, dude! And I think the confusion comes into play because “tiger-kneeing” an instant air move is a way to get it to come out quicker, essentially.
Instead of jumping forward and then executing a fireball motion when you’re already in the air, which would have your move come out however many frames later it takes you to perform the motion, you do a tigerknee input and hold that up-forward motion at the end of it a split second before hitting the corresponding button.
What happens if you do it right is you’ll jump and immediately perform the instant air move because the motion for it was already buffered in before you jumped (instead of after, which again would result in it coming out later in the jump).
Hope that makes sense!