What does the term bulb ramping refer to and what does it bring the world of time-lapse photography?
It seems you need a specific type of intervalometer for bulb ramping. How do these differ from the average intervalometer?
Bulb ramping, or bramping, is a means of automatically adjusting exposure settings to maintain a specific exposure value (EV) throughout the duration of a time-lapse sequence. Bulb ramping intervalometers can be simple and cheap, or complex and expensive, depending on the results they can provide. Cheaper ones, and many DIY projects that you can follow to build your own, tend to produce fairly apparent jumps when exposure settings are changed, resulting in less-than-ideal results when a time-lapse sequence is stitched together into a video. More expensive intervalometers that offer bulb ramping capabilities tend to produce much finer adjustments over more frames, greatly reducing or eliminating visible jumps in exposure value in the final video.
Its somewhat possible to achieve bulb ramping with a normal intervolometer and automatic settings in a camera body. Things like auto ISO and a priority mode will usually achieve some degree of bulb ramping...but the results are often unpredictable. Using automatic and priority modes sometimes limit your options as well...such as only outputting JPEG images, or being limited in how much "ramping" can occur, etc. If you want the best results, buying or building a high quality bramper that supports fine adjustments over the duration of a time-lapse sequence will be necessary.