Speed intervention allows for MCP control of the speed while the FMC remains engaged in VNAV. It has basically the same function as LVL CHG.
Personally I only use the intervention in two situations.
1. When doing a VNAV idle descent and I'm messing around in the FMC with cruise altitudes, altitude restrictions or speed limits. Basically anything that can have an impact on the FMC commanded speed in VNAV. When using speed intervene, the A/P holds the select mach or IAS. So i can execute in the FMC without worrying that the AP will suddenly dive like crazy or go into energy conservation mode.
2. For speed control while flying a VNAV approach. The intervene function will only allow for actually speed control when in the approach segment of the coded approach or when you have flaps out (VNAV PTH). Otherwise it will go into retard idle descent. (VNAV SPEED).
If you have flaps out it will go into VNAV PTH with manual speed control via the speed intervention window 100% of the time. Without flaps selected, it's hit or miss depending on the programmed approach. I've asked a few people at what distance along the approach the FMC will actually transition into VNAV PTH and nobody knows for sure. I've seen it happen at around 10nm LNAV track, but sometimes already at 15 or later. Someone care to explain why?