I have had the concept of overtones explained to me before, but I didn’t really understand it. Could someone take this up – and talk to me like I’m 12. 
The sound of a whistle is a vibration in the air that’s generated by air going over and under the blade. The pitch of the sound is related to the wavelength (shorter wavelength = higher pitch). The wavelength is determined by the effective length of the tube. The longest wavelength is with all the fingers down, and as you pick up fingers, you’re effectively shortening the tube, thus raising the pitch.
The longest wavelength is twice the effective length from the window to the end of the tube; this is the fundamental wavelength of the low-D note. However, the tone is not a perfect sine wave. Other wavelengths are “allowed;” these being the fundamental divided by most integers. So, while the low-D is the dominant sound, there are also these higher notes, “harmonics” or “overtones.” (The first few of these are second-octave D, second-octave A, third octave D) They’re present in varying degrees, which is one thing that determines the different characteristic sounds of whistles.
By overblowing, you’re damping the fundamental, thus the second-octave D comes out. Lifting the index finger damps the fundamental further, which is why it’s easier to hit the second-octave D and why it sounds a little different.
Hope this helps.
So does that account for why, if I push my Overton Low D in low E and D I get what almost sounds like a whistle or buzz? I almost feel like there is a burr somewhere in the D hole or the E hole causing this sound. BUT maybe its just an overtone I’m not used to since I’ve only recently received this Overton Low D.
Comments?
BillG
“So, while the low-D is the dominant sound, there are also these higher notes, “harmonics” or “overtones.” (The first few of these are second-octave D, second-octave A, third octave D) They’re present in varying degrees, which is one thing that determines the different characteristic sounds of whistles.”
Awesome explanation Chas. That really does make sense. 
Some players, and especially flute players, will intentially force overtones, usually by hovering near the octave break. Varying effects can be created, but the most useful is probably as a means to strengthen the lowest notes of low whistles and flutes. There is a point where you can reach nearly equal upper and lower octave at the same time.