If you need to "lower your dead key" then you are overdriving your amp, causing distortion (at RF, this means unwanted nasties such as harmonics and spurious mixing products) that will interfere with other services such as TV reception. A "linear" amplifier (most backyard amps are anything but linear) will amplify different power levels by the same multiple when operated within its limits. For example, a 1000 watt rated linear amp with a gain of 10:
0.1 watts in gives 1 watt output
1 watt in gives 10 watts output
5 watts in gives 50 watts output
33 watts in gives 330 watts output
and so on. If you give it 150 watts in, it will be pushed beyond its limit and distort. A good amp (a proper commercial design, such as a Harris, Barrett, etc, not a cr*p backyarder "pillbox") will have an ALC circuit to limit gain and warn the user. They are specified to the "1db compression point" - in the previous example, that means the maximum input would be about 125 watts, which would limit the output to 1000 watts with RF distortion limited to about -30dB, which shouldn't worry the local TV reception too much.
The quote from shadowstorm is basically right. Some simple maths will confirm this.
Speech or music audio is symmetrical - it goes equally up and down, in other words equally positive or negative.
In an AM transmitter, when it goes positive, it boosts the carrier, and when it goes negative, it reduces it.
You can't see this on a power meter - it's happening too fast to see with a meter - you need at least an oscilloscope, but preferably a spectrum analyser to see it (especially if you overdo it).
On the negative side of the cycle, the limiting factor is cutting off the carrier, this happens when the negative cycle audio power equals the carrier power. Any more than this, it gets ugly real quick. This is our "100%" limit. Since the positive cycle is opposite and equal to the negative one, 100% positive will double the carrier peak VOLTAGE at the 100% point.
Since doubling the voltage doubles the current, and power equals voltage times current, then it follows that the peak power (the PEP wattage) will quadruple. Any "swing" above this will be over 100%, and because the negative is the equal opposite of the positive, the carrier will be cut off, resulting in distortion and splatter.
This limitation results in wasted power, not a concern for a 4 watt or even a 100 watt transceiver, but it is an issue for a 5000 watt+ AM broadcast transmitter. To overcome this, they operate on reduced carrier and go over 100% on the positive cycles only. This can't be done with a simple "swing kit", it requires complex circuitry. The Harris broadcast transmitters use DSP (digital signal processing) modulators to achieve this, and cost five or six figure dollars. Have a look at:
http://www.broadcast.harris.com/product ... =WWWDXDEST
CB Swing kits are snake oil material. They might sound "loud" on air - most people associate loud with distorted, if you turn up a $5 portable radio or a bedside alarm clock radio they go "loud" distorted.
Add your "pill box" backyard amp and you've added spurious to the list - at best this might get your coax pinned by another local operator or irate TV viewer, at worst you might knock another service off the air in an emergency or get a visit from the ACMA (or your local equivalent if you are not in Oz).
If you are going to run over the legal limit, at least keep your signal clean.
Cheers