There are a few types of dynos, inertial dynos, and loading dynos..
Inertial dynos are simply a large drum of known mass, which is accelerated, then calculations are made, atmospheric corrections made, and HP, TQ is presented,
This
Dynojet is a simple inertial dyno..
Basically it is simply for indicating HP & Tq and is not a very effective tuning tool.
Loading dynos are dynos which load can be applied as the car is "driven" this allows tuning under many load conditions..
You can tune part throttle, light load, part throttle heavy load.... full throttle light load (going downhill) etc..... you can tune any area of a map only with a loading dyno
Load can be applied via Eddy current, water brakes, large disc brakes.. and other methods
This
Dynojet shows a loading unit attached...
A pretty good write up on dynos
Dyno Wiki
Additionally especially with a turbocharged car, the dyno facilitie should have some pretty serious fans on hand to cool the engine get air moving over the intercooler..
i.e. on a water-cooled twin turbo we had on the rollers recently, we used a 4' dia fan in the front of the car cooling the radiators.. a 440v squirrel cage blower feeding each inter-cooler in the fenders...
Also don't expect exact repeatability on dyno pulls.. results vary hour to hour, pull to pull..