It is a natural reflex to use less air when you don't like what you hear or when you are unsure if you're playing the right thing or not. If you don't like your tone on a bad day, you'll tend to want to "hide" it by using less air and playing softer. The problem with that is that you only make things worst and end up in a vicious circle of less air-tension-forcing-bad tone-even less air- etc...
If you don't like your tone for any reason, you better just play out as if everything was normal. If some note disturb you, don't hide! Use a normal air flow or more air if it's appropriate. That way, it won't get worst at least and it'll get better eventually with a bit of patience.
If you are sight reading or don't know the piece very well, using less air will not help you. On the contrary, it'll only make you miss more notes and make more mistakes which will further affect your self confidence. Even if you aren't sure, you are probably playing the right thing! So play with confidence. If indeed, you're playing wrong notes, you'll fall back on your feet faster with more air.
So don't let a few bumps on the road affect your playing. Play out no matter what and you'll always sound strong and confident.
At last, the first five numbers of the progressive method vol. 4 are now available to download on the french horn method page. It took a bit more time then I expected but I hope to upload more numbers soon now that baby is sleeping better! I hope you enjoy this new book!
I am associate principal horn of the Singapore Symphony Orchestra and the author of the progressive methods. I'm happy to share my experience as a horn player and teacher with you.