It's kind of an individual thing and mine vary depending upon how I'm feeling the day before a race. In general though it involves going sort of hard at interval lengths much shorter than I'd normally be assigned for the efforts I'm given. Instead of 20 or 30min tempo i'm given 8min at the lower end of the zone, instead of 5min vo2 things they're 1min. Just enough to remind my legs they're supposed to function, as typically the day before openers I do an easy spin.