In a slot machine, a player wins if the actual stop on a machine corresponds to the symbol combination determined by the computer chip pre programmed into the slot machines. There is no exact way of telling the slots odds of winning on a sot machine as this computerized chip is programmed at the factory producing them. The casinos in any way can not change the odds on the machine unless it replaces the chip and replacing the computer chip is not permitted by strict legal regulations. 

There is no way to 'tighten' the slot machines as the numbers generated by RNG, which are the slots odds, are always irregular in sequence. This means that the numbers are picked at random each time the reels spin. The chances of the player for winning are the same each time. The winning of the player never depends upon the other spins and there is no correlation between the spins. It is a matter of chance and sheer luck of the player to win in a game of slots.

The slots odds are determined by the RNG, which lays out the payout percentage for each slot machine. The gaming commissions tests that the machine pays out a minimum percentage of the money deposited in it. This payback is often 90% to 97% in most of the casinos. Even if the casino earns mere 5% of profit, in case the payback is 95%, one can easily see that a casino never lose in any case.