Source: Loser On Loser Solutions By Daniel Bertrand
LOL can have many different meanings. When you play a bridge hand, it can stand for Loser On Loser. Let’s see how this technique can increase your chances of making some contracts.
On the following hands, assume that you are playing in a team game (i.e. do not try for overtricks, just give yourself the best chances to make your contract). You are West.
1) 
Due to your great bidding, you reach a contract of 6
. North leads the 
Q. You could try to ruff all your diamonds and rely on the heart finesse. Is there anything better?
2) 
You avoid 3NT and get to 4. North leads the 
A and continues with the King. You could ruff and pull trumps; if they break 3-3, you will be OK. Can you improve your chances?
3) 
You are in 4 after North opens with a weak 2. North leads the K as South follows with the 10 (Standard Signal). What is your plan? Perhaps you could ruff your third heart with the 10 hoping that North has the spade Jack, or ruff with the Q and hope that you can pick up trumps without a loser. Do you have a better plan?
4) 
You reach 6 again. North leads the 
10 and South plays the Queen on dummy’s Jack. If South has the diamond King, you will be OK. You could play the A first in case North has a singleton King; then if the King does not appear, go back to dummy and play a diamond toward your Queen. Can you improve on that plan? Once you decide to pull trumps, you will find that South has both of them.
Solutions
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1)
Due to your great bidding, you reach a contract of 6. North leads the Q. You could try to ruff all your diamonds and rely on the heart finesse. Is there anything better?
On the first hand, the simplest way to make your contract is to play a small heart from Dummy on the opening lead. Your Right Hand Opponent (RHO) will probably win the Ace and then you will be able to throw a second heart from Dummy on the K. If your RHO plays a heart at trick two, you win your Ace (no need to try the finesse), pull trumps and throw the remaining heart from dummy on the K. Then you have the rest. By throwing a Loser On Loser, you were able to create an extra winner: the K.
2)
You avoid 3NT and get to 4. North leads the A and continues with the King. You could ruff and pull trumps; if they break 3-3, you will be OK. Can you improve your chances?
On the second hand, when the defense plays a second round of hearts, it is better to throw a small diamond from your hand. If the defense plays a third round of hearts, throw another small diamond from your hand. Now the defense cannot continue hearts since dummy can ruff high. The defense will probably shift to a diamond; you can win and try to pull trumps. You will succeed if they break 3-3 (36%) or 4-2 (48%). If you ruff the second round of hearts, you will only succeed if trumps break 3-3. (So you increase your chances by 48%). By throwing a Loser On Loser twice, you hopefully have kept trump control.
3)
You are in 4 after North opens with a weak 2. North leads the K as South follows with the 10 (Standard Signal). What is your plan? Perhaps you could ruff your third heart with the 10 hoping that North has the spade Jack, or ruff with the Q and hope that you can pick up trumps without a loser. Do you have a better plan?
On the third hand, you should win the A and play another heart. North will win and try to get her partner to over-ruff the dummy by playing another heart. But instead of ruffing, you play a small diamond from dummy! Since North has 6 hearts, it is quite likely that South has at least 4 diamonds. After North wins the third round of hearts, she will have to shift to another suit to avoid giving you a ruff and sluff. You will win that trick and then you can play your A (if you still have it) and a small diamond. Then, when you regain the lead, you can try to ruff your small remaining diamond. By throwing a Loser On Loser, you are able to take a much safer ruff.
4)
You reach 6 again. North leads the 10 and South plays the Queen on dummy’s Jack. If South has the diamond King, you will be OK. You could play the A first in case North has a singleton King; then if the King does not appear, go back to dummy and play a diamond toward your Queen. Can you improve on that plan? Once you decide to pull trumps, you will find that South has both of them.
The fourth hand is more challenging. After winning the first trick with the A, you should pull trumps. Then you play the K; a small heart to dummy’s Ace; then you ruff a heart. When you ruff your last club, you reach this position with the lead in dummy (East): 
Since the defenders started with seven hearts between them, and three rounds have been played, one of them must be out of hearts. So you play the 8 from dummy and hopefully South will not be able to follow suit. When South does not produce a heart, you simply throw your small diamond and the slam is yours (Check it out). By throwing a Loser On Loser, you have end-played your opponent. (If South produces a heart higher than the 8, you ruff and will need a winning diamond finesse).
As you can see, playing a Loser On Loser can be done for many different reasons. There are many more: to prevent one particular opponent from gaining the lead, to establish a side suit, to rectify the count for a squeeze, to execute a scissor coup, etc.