Suppose, you are given a key to open a lock. What would happen if the key is damaged or half of it is missing? Would the lock open? No. What if you are trying to insert the key upside down? The lock will still remain closed.
Similarly, salat is the key to Paradise. In order to open the doors of Paradise, we must make our salat proper. If our salat is totally opposite to the ways of Prophet Muhammad (peace be upon him), then it is as if we are trying to put the key upside down. And if we don't have concentration and devotion in salat, then it is as if we are planning to use a damaged key to open a very secured lock. Finally, if we do not complete our prayers with all the necessary actions, then it is the same as using a key which is missing parts or half broken.