Hi rjclark, you should be able to to do this by:

1. setting the SPEED knob of the tremolo (or rotary) to your desired slow speed. 

2. Set the ModSource knob to Envelop or ADS

3. Turn the Rate knob (should be for sensitivity now) all the way up to 99

4. Strum a chord and set the SMOD to achieve your desired fast speed.

5.  Reduce the Rate (Sensitivity) to get your desired slow/fast sweep time. 

Does this help?