For some people, the "not saying no" part is a slight red herring. The principle is that you teach your child the correct behaviours, modelling them constantly, and then monitor and intervene before they get to a point of doing something that you would actually want to say no to. The more you do of that, the less the child is actually likely to do anything that you'd want them to stop.
In parallel to that, you shower the child with a base level of lots of attention, praise (etc). So, if they do veer towards "unwanted behaviour" the thought of parental disapproval (and the loss of the warm attention and praise) is enough of a disincentive, and No is not necessary.
In order to be properly effective, it's kind of a full time method, involving all the monitoring and intervening I describe above or perhaps better described as a "total approach", like a mindset rather than a strategy.
Sometimes is misunderstood to just mean - all you have to do ignore child's naughty behaviour and just focus on positive. Which can be effective but not always, particularly if you don't want the naughty behaviour at all. Also sometimes very understood to mean "I just don't say no, it limits my child's freedom" which can be problematic.
In general, not noticing a child's good behaviour until they're doing something wrong and then expecting them to respond to someone saying "no"! isnot always effective, basically as they wont necessarily care it's a negative response and might enjoy the fact that someone is noticing them.