The meaning of iambic explained
Generating code with an iambic key needs 2 components. Number one is the key itself like the BD5. The key can be thought of as simply 2 switches, a right switch and a left switch. These switches are turned on by pressing the right or left paddle. With an iambic key each switch can be turned on separately or both switches can be on at the same time. In a non iambic paddle only one switch can be on at a time.
The second component is the electronic keyer. The electronic keyer sends the Morse alphabet not ASCII. The electronic keyer's output mimics a single switch. It is either on or off. The keyers output connects to the radio's key jack (not the paddle input) and turns the transmitter on each time a dit or dah sent. The electronic keyer can be a stand alone piece of equipment or it can be built into the radio. The radio or keyer usually provides an audio tone, called a side tone, so the operator can hear the Morse characters that are being sent. The audio tone is not sent over the air. It is only provided as an aid so the operator can hear the code as it is being sent.
With a single switch key, called a straight key, the Morse characters must be sent manually. i.e. To send an A you must tap the key once for a short period (dit) and once for a long period (dah) thus making the Morse character A. For the character Z, dah dah dit dit, you must tap the key 4 times.
To send the letter Z with an electronic keyer and paddle you need only press the paddles 2 times, once for the 2 dahs and once for the 2 dits. The fact that the number of presses is less with the paddles allows one to send faster with the same amount of effort. With an electronic keyer as long as the left paddle is being pressed the keyer will continuously send dits. When the right paddle is being pressed it will continuously send dahs.
Another advantage, and probably the most important, is the fact that the electronic keyer makes the dits, dahs, and spaces in between with exactly the correct length and spacing to create nearly perfect code.
So now you ask what does iambic mean. Lets take the letter Q. dah dah dit dah. From my previous description you would think 3 presses of the paddles are necessary. In a non iambic keyer this would be true but with an iambic keyer only 2 paddle presses are necessary. The way this is accomplished it to hold the dah key for a time length that would send 4 dahs then after the 2nd dah is sent simply tap the right paddle to insert the dit. WahLah......only 2 presses of the paddle.
Being able to tap the second paddle while the first paddle is being pressed is called iambic operation.
Using an iambic paddle and keyer allows you to send nearly perfect CW at a faster speed with 1/10 the fatigue factor as a straight key.