The tremolo effect causes your volume to rise and fall continuously, creating a swirling effect. If you turn it on fast enough, you'll sound like a machine gun. Here are some examples of how I like to use this effect:
I used the Opto Trem in my pod, with the speed set to a 16th note, wave = 33%, mix = 52%. The tabs for Emmanuel can be found here.
I sometimes use the effect for upbeat songs to create a feeling of suspension in the music, like in the example with Did You Feel the Mountains Tremble.
This effect comes in handy during the slow songs as well. I usually add some delay (digital delay, dotted 8th note, feedback = 39%, mix = 43%) in, and the result is a 'warbbly' tone with an airy feel about it, one that sound a little more like an organ. It's a little hard to catch with the music, so I've played it alone at the end. It works with a cleaner tone as well, as I've used some distortion in the recording.