There are two types of chokes: blood choke and air choke. The consensus seems to be that blood chokes are safer than the air choke or breathplay, however, none can be made completely safe. All three are related in that they reduce oxygen going to the brain, which causes a sense of euphoria.
You should establish some kind of non-verbal safeword, in case the person can't speak, such as tapping on your arm twice.
Watch your partner at all times. Reportedly they can pass out without much warning, and it's very important to release pressure immediately if they pass out.