The Haas effect is a byproduct of the method that our brains and ears use to determine the directionality of sound. A sound coming from your immediate left will be heard by your left ear first, then the right. Your brain interprets the time delay between the two separate sounds to determine the specific direction. By creating two sounds that play in opposite ears in very quick succession, the brain is tricked into hearing one large, expansive sound that appears to be coming from both sides at once.
A digital delay effect with a pan control is the simplest way to exploit the Haas effect. The unit must support 100 percent wet output and very short delay times, as low as 2ms is ideal. Removing the dry output prevents phase issues cutting out part of your sound. Route the audio signal to two delay units. Pan one to hard left and the other to hard right. Set the delay time on one unit to zero and the other to 2ms. As you play back your sound, adjust the delay time of the second unit up to around 30ms. Up to this setting, you will hear the wide, stereo spread created by the Haas effect. As you approach a delay time of 30ms, you will start to perceive the left and right channels as two different sounds.
When playing back your sound, automation of the delay parameters may also be useful as it will allow you to demonstrate how the spacial effect is created. Start with a plain, unaffected mono sound. Play it through the headphones or speakers to your listeners as a reference point, and then play the sound with a long spacial delay. The listeners will hear the left and right repeats of the delay as two distinct sounds. As you shorten the delay time to a length that will confuse the brain's spatial understanding of the sound, it will start to appear as one "expanded" stereo image, appearing to come from both sides at once.
The Haas effect demonstrates how the brain determines directionality of sound by measuring the time delay between two distinct instances of it from the left and right. As the delay between the two sounds is reduced, the overall effect begins to resemble a sound coming from the front as if the two ears are receiving the sound at exactly the same time.