"under load" means that the bushing side of the arm, which has a bolt through it where the inner metal cylinder meets the subframe, is tightened with the full weight of the car on the suspension. There are two ways to do this: put the car on the ground and then tighten the bolt or, with the car in the air, put a T stand under the steering plate and raise the corner of the car to what normal ride height should be.
The bushings have to be set under load because they are ride height specific -- if they're torqued with the wheels dangling, there will be little upward movement and the car will have an unpleasant ride, not to mention wear the bushings out very quickly.
best, whit