There's no
technique to speak of, Javier, because you're simply compressing a spring when you pull the lever down. The barista simply pulls the lever down, holds it down for however long the preinfusion is to last, and then lets go of the lever.
With coffee in the basket, the lever rises slowly under spring power as the extraction proceeds*.
The resistance the lever offers is not very great. No sumo stance required.

A sylph would have sufficient arm strength to operate the Lusso. Because the machine is relatively heavy and has a low center-of-gravity, it does not need to be steadied while the lever is being pulled down; so one could always grasp the lever handle with both hands. Moreover, excellent results in the cup are attained with a very light tamping force, so no arm strength is required for that either.
While the shot extracts the barista's hands are free to do other tasks, such as steaming milk, which can be done while the shot extracts without any adverse effect upon the extraction; intra-shot brew temperature remains constant because the brew water is already in the piston chamber. Nor is inter-shot temperature stability adversely affected by steaming; with the large 3 liter boiler and 1000W heating element, recovery after steaming is almost instantaneous.
Regards
Timo
*CAUTION: One should never simply let go of the lever when it has been pulled down while the basket is empty. With no coffee in the basket there is almost no resistance, and so the lever handle will spring back up with great force.