Alesis 3630 Reference Manual page 26

This limited, amplified signal has a much higher average level than the
original signal. This is why limited signals can "jump out" at you and have
more punch. Commercials, for example, are often heavily limited so that
they have as high an average signal level as possible. Radio and TV
stations also use limiting to cope with the medium's limited dynamic
range.
If the limiter's clamping action occurs abruptly—in other words, the
limiter goes from no limiting to full limiting at the threshold point—the
sound's output level will not increase despite changes in input level. This is
called a hard knee response and is often used to eliminate loudspeaker or
amplifier clipping.
With a soft knee response, the limiting action becomes progressively
greater past a certain point until it eventually flattens out and clamps the
signal fully, just like a hard-knee limiter. This tends to produce a smoother
limiting sound that helps smooth out an instrument's dynamic range.
17