I think @B-26 is referring to an add-on speed control, not a router with a built-in speed control.
Motors and other inductive loads work differently than resistive loads like incandescent light bulbs and (other) heater coils. Simple dimmer switches are designed to work on resistive loads. They add a variable resistance inside the switch which redirects energy that would go to the lightbulb and converts it to heat instead. These work with old incandescent bulbs, but not on newer LED bulbs that use much less current. The resistive load inside the dimmer means power flows through even at very low/off settings, and this low current is often enough to cause LED’s to glow even when the dimmer is turned off.
The sort of control that @B-26 is talking about uses a Silicon Controlled Rectifier (SCR) called a triac to turn the power completely on and off several dozen times a second. These are sometimes referred to as “chopper” controllers. This allows motors to run at variable speed, but can have a negative impact on the power (torque) the tool can actually put to use, since it’s generally the higher power portions of the energy wave that get “cut off.” This means that below a certain power setting/RPM, the tool becomes very easy to “stall.” Newer LED-compatible dimmers probably also use a triac (I’m not certain), but I wouldn’t expect it to be robust enough to handle the current for running a spindle.
The controllers built into routers generally start with the triac and add a PID feedback mechanism so that if the motor slows down, power is added back in to the circuit. This keeps the bit spinning at close to a constant speed even as the load on the tool varies (starting and stopping a cut, or hitting a knot in a piece of wood).
If you’re comfortable playing around with mains voltages and some programming, @vicious1 has a PWM AC Dimmer board in the shop that can form the basis of a PID-loop controller as documented in the (very long) PID - Hardware Solution Needed for a Software Fix. Note that this is not a turn-key solution - it requires an additional Arduino Nano on a daughter PC board and several components to be added to your spindle. I don’t know the rotozip well enough to know whether there’s space for the sensor in there somewhere. Having said that, I’ve got it working on the DW660 and I like it a lot.
Another commercial solution available is the Super-PID, but this is aimed more at high-end routers. It requires the addition of similar components to the @V1 kit be added to the spindle, but all the other circuit assembly and programming is done (and not user modifiable).