It looks like a fairly simple mechanism. You lift the nozzle and the water stops flowing and redirect the nozzle over a bucket, then lower the nozzle again. This should start the water flowing again and empty water from the aquarium into the bucket. Once you have removed however much water you want, lift the nozzle again to stop the flow and then redirect it back over the filter and lower it again to get water circulating back into your filtration. Refill the aquarium.
You might want to refill the aquarium before you press the nozzle back down to start the filter flowing again. If there isn't enough water in the aquarium once you restart it, it could be running dry.
Could you post a video somewhere like YouTube of what's happening and link it here?
If its not working as expected I would presume its faulty and should be returned. You could contact the manufacturer and see if they can troubleshoot a solution, and if not speak to whoever you purchased it from to organise an exchange.
Are there any fish in the aquarium? Or did you run the aquarium with no fish first to make sure it was working as expected before getting a fish?