most dynos just use a ramp rate, a given speed that it will let the car accelerate, and use eddy current resistors (i think) to oppose the torque from your wheels, therfor, load is generated.
as a general rule, most tuners use 1:1 ratio (4th gear in just about every 5 speed manual), so you should set the ramp rate similar to what the car accelerates on the street in 4th gear... although most tuners will just use the same rate on every car, and maybe give it a long, slow, hard run to redline after they're finished to check everything is ok
point is, dynos do generate load, and can generate alot more than your car will ever see, dynos can load up your car, as much, or as little as you want, and at any given load/rpm
thats for like dynodynamics and things like that, not american mustang dynos, which just use a big heavy roller and measure the time it takes to spin it up. *might be thinking of another brand*