ORIGINAL: Lilredmachine

It's a flux ring, as pointed out.

They are a method of focusing, oversaturating the magnetic flux/field so that it is held in the can and onto the armature, in a way they are like another form of gearing. Depending on the material used (there are many different types, most are a complex alloy concoction containing some, or all of high-silicone iron, armco magnosteel, transformer steel, semicarbon 29K steel, etc.) it can have a different effect. Mostly they are used where greater torque is required (hence 550s) and removing this ring will (in extreme cases) give 30% higher RPMs, 20% less torque, 10% more power, will degrade the runtime, increase the current draw, decrease the efficiency and therefore increase the heat released by the motor. The eventual effect is a motor that will fail a lot earlier than one with a flux ring. Magnetic field strength and current are two key factors in deciding a motors torque, by controlling it and focusing it you can change how a motor performs.

Think of it as a direct way of dropping a couple of teeth on a pinion. Like most things, they are there for a reason. Do not assume you will decrease the heat produced by an E-maxx Titan by removing the flux ring and fitting a heatsink.