Most diagnostic nerve blocks are the same, in my experience.
The anaesthesiologist injects a numbing agent, like lidocaine or something similar into the painful area that you pointed at.
If they hit the right spot and your pain is nerve related, then you should have partial or complete relief from your pain for a short period of time, up to two hours or sometimes more, but rarely more than a day or two.

If the diagnostic injection worked, then they can try another injection using a steroid added to it.
For some incredibly lucky people, one or two steroid injections can sometimes permanently reduce their pain. But that is not the normal outcome for most ACNES sufferers. However, it is definitely worth a try for you.