The second part of the ramjet is called the diffuser. Air that comes through the inlet is moving very fast, which makes it very hard to burn fuel. The job of the diffuser is to slow the flow down in an efficient way. By the time the air has passed all the way through the inlet, it is traveling slower than Mach 1. This is important for the diffuser to work properly. The diffuser is essentially just a pipe that increases in area as you move along it. Because the flow moving slower than Mach 1, the increasing area causes the air to slow down.