First, a caveat: I don't employ this technique much except in certain circumstances. In the absence of an adjustable trussrod, I prefer to plane and refret or simply replace a board to true up an errant neck. Working with wood as it is yields much better results than forcing it where it doesn't want to be. I got one of these tools in about 1972, and used it extensively on all kinds of things end eventually found its limitations. 

Before electric heating blankets and pads, there was this tool:

It's a very rigid steel box with a reliable true surface, with electric heating elements inside, and an on/off switch or a rheostat (pictured). Through that little hole to the right of the rheostat is a small bulb that tells you if it's on or not. It came with a pair of heavy padded U-clamps and one for clamping from the soundhole onto the fingerboard extension. This photo is purloined from somewhere in cyberspace, it's not mine, but it's just like mine. My rheostat died almost as soon as I got it, and ever since, it's only had an on/off switch. 

The idea was to heat the neck, soften the glue joint between the board and the neck, force the neck into the desired shape under that heat and pressure while the glue is soft, then turn it off and let it cool still clamped. The glue would reset, the two surfaces that the glue bonded would be in a slightly different new relationship, and the neck would stay that way. That was the theory. In practice it took some farting around to get correct results.

In many instances, this works like a charm. Martins work well, as do many Asian instruments glued with epoxy. Gibsons and Guilds were iffy. Older guitars of any make don't usually respond well because their glue is no longer softenable. Water-based glues in general do best if they still contain a bit of water or if the wood around them does. These devices will make crispy critters out of any plastic fingerboard inlays, too. Very fragrant.

Here's a crude sketch of the heater (light blue thing) in full idealized contact.

 Closer up below:

The heater lays right on the frets, which keep the wood from scorching. In fact, the real thing is shorter than most boards, but long enough. 
 

The heating box is dead true, and when you rest it on the frets, you often see the expected gap, which if all was well, you would believe it could fix. But simply clamping the neck to the heater alone is usually not sufficient. 

Instead, I put a small shim (such as a piece of cardboard) between the heater and the first fret, to overbend the neck a bit. After the whole thing cools and the clamps come off, there will be a bit of springback, and the desired relief will be there.

This takes care of the most common problems. A twist can be dealt with by strategic use of shims in various places. A lot about this tool involves trial-and-error. And a willingness to punt and use other methods.

A simple light neck reset can be done too. The heating element runs through the whole box, but you can control the degree of heat in desired areas by using shims. I made a long tapered that made sure the glue joint between the fingerboard extension and the top was heat-softened, but not the rest of the board. The shim does two jobs: insulates the board, and pushes the neck back and down. This works marvelously well on guitars needing just a bit of a reset. 

The shape of the wedge is, of course, exaggerated for this illustration. The actual one I've used for years is made of pine. In this procedure, the fingerboard extension moves ever so slightly back from the soundhole. No touchup is required, but it moves enough to re-angle the neck a bit.

Sometimes, when dealing with a typical bowed neck, I'll put a shim under the fingerboard extension to push the neck back as it heats the misshapen part of the neck (below). This is a lighter "reset" yet, but it helps. The heating device is extremely strong and rigid, so you can clamp like mad and it will stay straight. This gives it a distinct advantage over silicone blankets and so on.

Here's a tool that I have never used:

However, I have a similar thing I made for this task from a huge bronze roller bearing many years ago. The bearing was sliced down the middle and has a porcelain handle on it. I heat it on a hotplate, and use it to soften the glue under the end of the f'bd when I'm preparing to remove a neck for a real reset. Again, I prefer to use the frets as a slight spacer to keep the metal off the wood, so I'm not a sales prospect for the thing above. But it's a good idea for a specific task. 

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One thing I still routinely use my big neck heater for is removing fingerboards from necks. Again, there are old glues that are simply not affected by heat (anything by Lyon and Healy comes to mind). But when all you want to do is get the old board off, even ancient glue joints can be coaxed into yielding with a little help from the heat.