How compareAndSet works internally in redis

spring-data-redis module contains RedisAtomicLong class.

In this class you can see

public boolean compareAndSet(long expect, long update) {

    return generalOps.execute(new SessionCallback<Boolean>() {

        @Override
        @SuppressWarnings("unchecked")
        public Boolean execute(RedisOperations operations) {
            for (;;) {
                operations.watch(Collections.singleton(key));
                if (expect == get()) {
                    generalOps.multi();
                    set(update);
                    if (operations.exec() != null) {
                        return true;
                    }
                }
                {
                    return false;
                }
            }
        }
    });
}

My question is why it works?

generalOps.multi() starts transaction after get() is invoked. It means that there is possibility that two different thread (or even client) can change value and both of them will succeed.

Is operations.watch prevent it somehow? JavaDoc doesn't explain purpose of this method.

PS: Minor question: why for (;;)? There is always one iteration.