Don't get it at all.
*WARNING-- Spoilers follow*
This movie is just a long, slow, bad version of Rififi. What is most bothersome is that instead of the crooks being cool and crafty, they are lame and blundering. No wonder the movie starts out with them captured or in jail-- I'm not surprised (and they ended up dead-- duh, saw that coming)! The dude with the mustache acts all cool and smug throughout the whole film and you just want to slap him because he's simply a fool. So many blunders from this wanna be street-smart gang.
"Hey I just got out of jail-- how about I buy a car with dirty money from a rival criminal?!"
"Hey, let's recruit the new gang member at a known hangout for gangsters!"
"Hey, we can poorly tie up the guard right next to the alarm button!"
"Hey, let's leave the rope ladder with our fingerprints all over it hanging from the roof of the jewelry shop!"
"Hey, let's steal millions in jewelry and have ONE possible buyer for it all!"
I could go on and on... (continuing with the fact that the tip of this "easy heist" came from a POLICE GUARD!).
The ONLY neat scene is where Yves Montand takes the rifle off of the tripod mount and shoots the alarm trip sensor. Actually his character is the only interesting character with any sort of depth in this entire movie. And at least he's not stupid enough to die by getting SHOT IN THE BACK while RUNNING AWAY!
Oh yeah, the inspector dude is an idiot too. "I know, I'll blackmail the bar owner in to giving me information. Oh, he told me to *beep* off. Now what do I do? I know, I'll haul in his son for leverage. Okay, now his son just killed himself..."
However, if the intent was for the protagonists to be a bunch of dummys, then mission accomplished!!! Great film!!!