I assume that it is simply an extra boost of power to aid acceleration. If I am correct, when is it used and why can't it be on permanently? I tried searching the forums but got nothing.
This page of the RailUK forum contains a discussion on it. The fullest description appears to be this from 'SansHache': The introduction of Boost/Eco mode evolved from energy consumption measurements performed on 390049 in 2007. Net consumption was compared for multiple runs on different routes over several weeks and it was observed that the consumption was lower when operating with one of the six Traction Converters isolated. This was not in line with the predicted performance, but the results were consistent. The trains were designed to be able to meet the PUG2 (140mph) journey times with one Traction Converter isolated. The tests in 2007 pre-dated the introduction of the speeded-up VHF timetable, so there was plenty of slack in the timetable. It was concluded that the trains were using more energy with all Traction available as they were frequently running ahead of schedule, requiring the trains to slow to get back totheir scheduled path. This effect was mitigated to some extent when operating with one Traction Converter isolated (5/6ths power). Virgin were keen to take advantage of the reduced consumption as they were planning to switch over to metered energy measurement on the Class 390 trains. The Traction software was modified to introduce a new "Power Saver" mode, that limited the train to 5/6ths power when all Traction Converters were operating. This did not affect the Tractive Effort at starting, and only came into effect at speeds above 45mph. This is set as the default operating mode, but the driver can select full power via a button on the Driver's Display screen. If one or more Traction Converters fail, the remaining Traction Converters revert to full power automatically. "Power Saver" and "Normal" mode were re-branded as "Eco" and "Boost" when the function was introduced across the Class 390 fleet (positive marketing). It was never possible to fully validate the potential energy savings as the VHF timetable was introduced around the same time as the Traction software was updated, bringing a significant change in the duty cycle. However, one significant benefit of operation in "Eco" mode was the reduction in peak current in the Traction Converters. This improved the thermal performance (and hence reliability) of the Main Transformer and the controlled rectifier power module
The Class 380 is also equipped with a braking boost mode introduced by DTG in the game, you just need to be able to resist a high level of G force and a bag next to you, just in case.
