Pedaling fast is one of the best ways to improve your pedaling efficiently. It helps to activate muscles that might not be normally participating in your pedaling, increases your ability to accelerate, helps your sprinting, etc. All of these skills are incredibly important for criterium racing, where fast pedaling, accelerations and sprinting are what it's all about.
I've found that the best way to do these workouts is on my rollers in my easiest gear. The goal is to keep the gearing as easy as possible so that there isn't any momentum in your cranks, that way you have to keep applying force in both legs the entire way around the pedaling stroke. The goal is to spin as quickly as you can without bouncing on the saddle (i.e. you want to force your legs to keep it smooth) Normally I spin as fast as I can for 3 minutes, recover for 2 minutes, then slowly ramp back up again for 1 minute, and repeat 5 or 6 times.
Prior to my crash I could get my cadence up close to 130 rpm, which isn't phenomenal, but still pretty good. Since my crash I hadn't been able to do better than 114. That's until last week, when I was able to keep my legs going at 120, and at times hit 122. After one year of seeing no improvement, things are finally starting to get better. Yet one more sign that the nerve and muscle control in my left leg is starting to get a little better. It'll be interesting to see if I can now get back on a pace of improving 1 or 2 rpm per week.