What exactly is back pressure?
Take the hose off of your vacuum cleaner and blow into it.
Now take your whistle and blow into it.
The difference is back pressure. The whistle has it; the vacuum cleaner hose does not.
Note: If your whistle is a Clarke original C, you may not notice much difference.
Backpressure is the measure of how much a whistle resists being
blown into, compared to other whistles. If a whistle is said to have high
backpressure, it will take a lot of breath control to keep the pressure
up enough to get a good tone.
Backpressure isn’t a good or bad thing. Everyone has a different idea of how much force you want to use to get the tones you want. You may find your taste in backpressure changes over time, and in the venues you play. For example, in a loud environment I tend to want something with high backpressure so if I ‘lean into it’ I don’t squawk (as much!).
Neither of those seems very clear.
As a newbie, here’s how I see it:
Have you ever taken one of those breathing tests where you blow into the tube to try to float the ball? That’s no backpressure.
Or, take a somewhat large empty tube and blow into it. Next, cover most of the other end and blow again. The second is more backpressure.
How this effect relates to the amount of or quality of sound you get out of a whistle is what they’re referring to as backpressure. Obviously the differences between whistles can be subtle a lot of the time.
If I’m wrong about this, let me know and I’ll edit this post. As I said, I’m a newbie myself.
Thanks,
Jason
Backpressure and resistance are related terms to describe how much air it takes to sound an instrument, and how hard the air must be pushed.
An instrument with low resistance takes a lot of air to sound; an instrument with high resistance takes very little air to sound.
An instrument with low backpressure requires very little force–you can exhale into it, and it will sound. An instrument with high backpressure requires a lot of blowing force before it will sound.
An example of low resistance and low backpressure is the Clarke original whistle.
An example of high resistance and high backpressure is the O’Brien whistle…you need a good strong blow, and it doesn’t take much air.
An example of low backpressure and high resistance is the M&E (Cronnolly) whistle…it takes a lot of air, and the air also has to be moved with a lot of blowing force.
On whistles with low resistance, you’re always looking for a good excuse to catch a breath. On whistles with high resistance, you’re trying to find a way to quietly get rid of all your left-over air.
Whistles with high backpressure tend to be very pitch-stable across a range of blowing pressures. On whistles with low backpressure, pitch can sometimes be “bent” up or down depending on breath pressure.
–James
Actually, let me clarify something after reading Tyghress’ post.
It’s not just the quality or amount of sound in and of itself, but it also includes things like going into the upper register and hitting the bell note. This is what I really meant by quality of sound.
I’m probably making things worse. 
Jason
I was trying to follow untill I read these 2 lines which seem to contradict - or maybe I don’t understand something:
if low backpressure takes less force, why does the M&E example with alot of blowing force fits? ;/
Good catch: I typo’d it and then didn’t catch it, sorry.
The M&E whistle has high backpressure (requires blowing force) and low resistance (requires much air).
Sorry for any confusion.
P.S. if you think this is complicated for whistle, be aware that these two terms are also used to describe the way flutes differ from each other.
–James
James and all,
Well put I, I feel I can now explain it correctly. Thanks…
WTA
Yah, thanks a million guys. I just recently bought an Overton low D. If I understand what you guys are saying, the Overton could be said to have a reasonable amount of back pressuer, right? 
I think that depends on if the buyer asked the maker to put some back pressure in or not.