It took me a while to find out how to do it, but this help file explains it pretty well.I was able to use it to provide about a gradual crescendo on the final chorus of a backing track, with a controlled fade-out at the end, and used my selection at least as a guide to where to place my control points (I used the inner control points as I wanted to amplify). I thus left the original volume the same and added new control points between the beginning and end of the track and moved them left-right-up-down to control the timing and volume adjustment of each. Of course, you need to have plenty of headroom to amplify without risk of clipping, though until you export, any overs are properly preserved in floating point.You can zoom out on the vertical Waveform view to -2.0 to +2.0 and exceed full scale (but in linear view you can only see values up to a maximum of 2.0, though they may actually be much higher). Note that the envelope is only applied to the samples used in Audacity's tools when rendering the output. If you ran the Compressor effect on it to bring the peaks to -0.0 dB, the peaks would max out at your volume envelope, not the true 1.0 level. (The nasty Hard Clipping Limiter would also follow your volume curve), which means it also follows it during fade outs when you might have been happy to let your peaks exceed the envelope. If you Mix and Render it will 'apply' the envelope and change the actual sample values that are passed to any effects you use, so the envelope line will be flat again and no longer follow the peaks, and you can then tame those peaks that exceed 1.0 (up to around 2.0) using the Compressor's look-ahead limiter. On waveform view, you can drag the inner handle from 0.5 to 1.5 (9.5dB gain) and in fact, I tested rendering then repeating this 9.5 dB gain a few times until the light blue (approx average level) was above full scale then used Amplify -30 dB to restore my crisp sharp peaks, which I suspected were preserved from zooming out in Waveform (dB) view.I think the Envelope Tool has a limited range of operation (the amount of gain or attenuation you can apply) but you could Mix and Render the track then apply more Envelope if you wish to produce a deeper fade, for example. It also appears to be pretty much logarithmic, unlike the standard fade in or fade out, which is a mode that Audacity lacks otherwise.This youtube video gives guidance on the Envelope tool but forgets to demonstrate how to use the inner control points to increase volume.This youtube video gives helpful productivity hints including use of the multitool.