I am going to assume that what you heard was supposed to be...
"If you eat 300 calories of tomatoes and cucumbers... it will only count as 150 calories."
Eating them before a meal and having them take away half of the calories is wrong and makes no sense. Which I assume you knew because you seemed skeptical of it. Good for you!
Getting back to my "maybe they meant this.." statement above... some foods are so low in calories that when you take the calories it burns to digest them... it's actually half of what the total calories were.
Get that? lol
Let me break it down...
Lettuce- 50 calories (totally guessing... just using as an example)
MINUS
Total amount of calories burned by digesting the lettuce- 25 calories
----------------------------------------------------
25 calories retained
So even though you ate 50 calories worth of lettuce... you burned off 25 of them by digesting it. Pretty neat, huh? It's technical term is called TEF or the Thermic Effect of Food.
So with all that said... don't worry about any of it. Your body always burns calories when digesting food. No matter if it is a cake or a carrot.
That's the basis of the "Negative Calorie Diet". But it's not long term.
Stick with eating 5-6 small meals per day. Each meal has a protein and a carb. And is eaten every 2-3 hours.