This IS an actual parenting technique, but when used correctly has NOTHING to do with trickery.
I think it stems from the belief that agreement (when possible) validates what the kid wants, thereby reducing the # of tantrums that an outright NO might produce. Your 'agreement' is also conditional, if applicable.
It becomes effective once your kid learns how to reason (I would say around 3ish) and would NEVER be effective across the board for everything, or for each kid. It's also definitely NOT for every parent.
So.... "I want to buy a toy" has a response of "sure - when we get home we can see if you have enough of your money saved and next time we come back we will bring your money and you can buy the toy"
"but I want to buy the toy now"
"I know you do. and it's very frustrating when you want something and can't have it. And you can buy the toy - all you have to do is (repeat condition from above)...."
Similarly.......
"I want 2 scoops of ice cream"
"yes you can can have a 2nd scoop if you eat 15 more peas"
"I want to watch more tv"
"Certainly - you can watch tv once your toys are picked up"
"I want the movie Shrek 3"
"Yep, you can have Shrek 3 if you put it on your birthday gift list and someone purchases it for you"
etc etc etc
It has nothing to do with trickery and would be HIGHLY ineffective if used that way.
This is not a technique I used a lot, but it does have some advantage if you have a kid that wants a "higher level of control" of her world. Especially in public I avoided MANY tantrums by using this technique (depending on the situation).
Obviously if something is an outright NO then you would NEVER want to say yes (or apply a condition in which you MIGHT give a yes), and this becomes an incorrect and ineffective parenting technique for that situation.