Okay, so you've been out on the (cold, wet, snow-lined miserable) roads over the winter months and put in the base miles. Chuffing along at 80% MaxHR for four hours is fine, but how can a HRM help you when you want to get a bit of speed in your legs?
As with the previous set of notes, I'll start this by saying that I'm not a coach; what follows is a digested reading of lots of information out there – lots of it on the net, and lots of it in Joe Friel's excellent Cyclist's Training Bible.
Basics first. To get that speed, your training needs to bring about physical changes in your body. The basic process for this is that you stress the body in a specific way, and then let it recover and re-build. Within this process it will adapt, and you will become more able to do the specific activities you're training for. As Rob has mentioned in his intro, both elements of this are equally essential – the work and the recovery.
You may be looking for a range of physical changes. Raising your lactate threshold enables you to maintain a faster, steady pace – for example in TTs or a road race breakaway. Better lactate tolerance enables you to put in harder efforts for longer – when you "go into the red" up a hard hill, or closing down a gap. These and other benefits come largely from physical changes to your muscles and blood delivery systems. If you want to know more, there are good books (and good coaches) out there.
To get those physical changes the work needs to be more intense than those winter miles. Exactly how intense will depend upon specifically what physical changes you're after, but the HRM offers you one way of measuring the level of effort. It's not perfect; over short periods change in heart rate lags behind change in level of effort, and heart rate will vary with temperature and other factors. But, as with the winter miles, the humble HRM can give you a lot of useful information for a lot less money than a power meter.
So, what sort of training can the HRM help with? Training at higher intensity often involves "intervals" – relatively short periods at high intensity with recovery periods in between. Usually the higher the intensity, the shorter the intervals.
Here's a couple of examples from my own experience. Many of my interval sessions are done on the turbo, simply because conditions are more controllable and also I reckon it's safer – when you're nearly passing out the last thing you want is oncoming traffic. However, I know there are plenty of people (better riders than me) who disagree with this, and you can do these sorts of sessions indoors or out.
A session I find really useful is one designed to nudge the lactate threshold upwards, which involves short (around two minutes) intervals above LT, separated by equal periods just below LT. A half-hour session (plus warm up / warm down) of this looks like this when you graph it…
Another session is based around short intervals which build up to something like maximum possible intensity, which is designed to improve lactate tolerance. Graphed, it looks like this…
In these, and other interval sessions, the HRM is a valuable tool in showing you what level of intensity you're working at. Not perfect, but very useful. There are plenty of types of interval training for which the HRM is of no use at all though. Training for sprints for example, which is about very short, very intense efforts, has to be done by feel alone (or with a power meter). And don't forget the importance of bike handling skills – good descending, and efficient riding in a bunch or rotating paceline – can be as important as your level of fitness.