Envelope followers are used to achieve this effect.
You can read about it here: Download Now
and here: http://www.sonnoxplugins.com/pub/plugins/products/TransMod-Product_Info.htm
I have SPL Transient Designer in my studio and it's one of my favourite dynamic processors on some instruments. I also use Eventide Omnipressor (black face) to achieve similar results in a "traditional" way.

The Transient Designer offers a completely new technology for level-independent shaping of the dynamic response of a sound: For the first time it is possible to control the attack and sustain behaviour of a signal in a very simple way! Unlike other dynamic devices, the processing is not goverened by the signal level but rather by its dynamic envelope, so all signals (loud and soft) are processed the same way.

With no effort you can shorten or lengthen the attack and sustain of all kinds of percussive signals, such as the bass drum, snares, toms, congas etc. to let them jump into your face or to flatten the signals. Booming toms for example are dried up in seconds — it’s almost like being able to change the amount of drum damping after the recording. Your idea of a kick drum sound is a kick drum in the words best sense? Just dial it in.

The same applies to virtually any other signal: Amplify or reduce the picking sound of an acoustic guitar, hold the sound of the strings longer, reduce the reverb time of a choir, actuate the piano pedal “electronically”, turn down the slap bass a notch or give it even more attack, etc.

Don't forget recording.org sells all the SPL products in the Pro Shop. You must however, be a member in order to buy anything and/or receive special member discounts.
http://recording.org/modules.php?name=Shopping_Cart&file=product&c_op=viewprod&prodID=49

(y)

Now I know what it is, I just realised I have one.

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Awesome, even when I'm using it wrong.

Just two knobs - i love simple things

Thanks for the responses guys! I'd love to have on in the future to play with but it's on a long list of gear I want. Maybe I'll look into a plug-in but I always prefer something I can touch.

Respectfully,
Christopher