Jon1270
Minister of Fire
If your saw is working right, the chain won't be turning when you're not pulling the trigger
...unless the fast idle is engaged, as it often will be when starting a saw.
...and you can't pull the trigger without your hand fully around the trigger handle to first push down the trigger release thingy.
...which helps, but doesn't absolutely prevent accidentally grabbing the throttle at an unfortunate moment. Chainsaw users do tend to grip that rear handle now and then, even when they aren't actively cutting.
Obviously the OP needs to get their saw fixed, but a little redundancy in safety measures is a good idea. Locking the brake is sort of like flossing your teeth, i.e. a habit that's hard to establish because it's not any fun, with benefits you never tangibly see no matter how statistically, demonstrably real they are.