3030s will help boost your threshold, sprinting ability, and anaerobic capacity.
While not being the most enjoyable session, the benefits are worth the pain, and in a lot of cases can make races feel less painful than the training and preparation.
This session is best done on the flat, or on a slight gradient climb. The flat is best, as it's a bit harder.
Your workout will explain the length of the effort(s) and suggest you do Zone 5 - Zone 6 for the "on" part of the 30 seconds, and Zone 1 for the "rest" part. You can experiment with the "on" intensity. We like the idea of accelerating hard as soon as your 30 seconds commences, and holding the power as best you can for the 30 seconds. How hard you go should be determined by just being able to complete all the reps. Eg: if you have sets of 5 minutes of 3030s, you can go a bit harder than in 10 minutes of 3030s.
The key to each 30 second rep is to think of a "charge" to the line, in that you have gone early in a sprint and you need to hold on. The 30 seconds rest isn't really adequate time to recover, so you'll be starting the next 30 second surge with a lot of lactic acid in your legs and arms (and everywhere else). This is the intention, and while it hurts, each time you do the session, you'll learn a bit more about your own best way to cope.
30 seconds at Zone 5 - Zone 6 can effectively be 5 seconds blasting up to speed, holding on for 15-20 seconds, and crawling home for the last 5-10 seconds. Try to average around Zone 6, but if you're loading yourself up with lactic acid and averaging Zone 4, that is OK. It is OK to crack in the 30 seconds, however if you crack and it's not the last one, make sure you keep fighting through and attempting to do the next sprints. The point is to be accelerating while you're full of lactic acid, and fighting through to the end of each sprint. The reality is, the 30 second sprints aren't really sprints, but more of a "surge" or "charge" that will emulate an attack or chasing an attacking opponent.
The "rest", you'll find it's best to keep the pedals ticking over, but there is no set power. Keeping the legs moving will help rid the lactic acid. If you freewheel for the 30 seconds rest, you'll find the next rep extremely difficult! The suggested Zone 1 or easy rolling simply means, just keep the legs moving. For the much fitter riders, you might find rolling along with a bit of pressure on the pedals will make your legs feel better, but don't put so much pressure that the next sprints will be affected. The rest/rolling easy between sets is just doing as little as possible while still ticking the legs over, as you get ready for the next interval of pain!
In a nutshell, your set is made up of 30 seconds sprinting, accelerating, charging, holding onto your speed, followed by 30 seconds of rolling the legs over. Take no notice of the power you're holding on this session, it is more about feel than it is about power.