A detuned bass
A Reese is two or more sawtooth oscillators tuned slightly apart, so they drift in and out of phase and the sound moves on its own. Filtered and distorted, it becomes the dark, rolling bass of drum and bass and neurofunk; slowed down, it turns up in dubstep, techno and trap. A good Reese sample captures that motion, so listen for how it evolves over its length rather than how it sounds in the first half second.
Getting it to work
Reese samples are usually long notes or loops. Pitch them to your key in a sampler, and if the low end wobbles too much, high-pass the Reese around 100 Hz and add a clean sub underneath. Automating a low-pass filter over a phrase brings out the movement. Many free bass samples come from producers’ sampler packs; check the verdict on each before you release.