Normally, you have a few chances to make your contract (like in your example, hearts 3-3 or spade finesse). Combining chances mean timing the play in such an order such that the failure of any chance to materialize still allows you to take the next chance. Even though you might start with a lower chance, it allows you to take another shot rather than ending there.
If you do the math, this comes out to 1 - (1-c1)(1-c2)...(1-cn) where the chances of success of each chance are c1, c2, ..., cn and assuming the chances are independent. This is larger than any single chance (or a strict subset of chances).
You are in 3NT. LHO leads a spade.
You have to win this (top hand is dummy, bottom is declarer and suit order is spades, hearts, diamonds, clubs from top to bottom).
You have 8 top tricks and need just one more without losing the lead.
Now you have three major chances:
- hearts 3-3
- diamond Q dropping in two rounds (diamond finesse is a practice finesse).
- club finesse
and a minor club Q dropping singleton.
You test for club Q singlton, no luck.
Now the correct play is to test diamonds first, then test hearts and then take club finesse.
Notice that if diamond Q does not drop in 2 rounds, you can still test hearts. If the hearts don't break 3-3, you can still take the club finesse: thus combining all your chances.
Imagine if you had taken the club finesse and it loses. Now you are down, without getting to test the diamonds or hearts.
Note that you have to test diamonds first. If you test hearts first, and then diamonds, then you are in the wrong hand and cannot take the club finesse!
Here is another example, where you have to take a finesse first.
You are in 6S. LHO leads a trump.
You have 11 winners and need one more. Now your major chances are
- heart finesse
- diamond finesse
- clubs 3-3
The correct order of play is to try the diamond finesse first.
If that loses, you can now discard a club from hand on the diamond A, play AK clubs and ruff a club, thus testing for clubs 3-3. If that fails, you fall back on the heart finesse.
Note that you cannot time the play in any other order. If you test clubs first, and it fails, you are now committed to guessing which finesse to take.
If you take a heart finesse first, then you cannot test for clubs 3-3 and are forced to fall back on diamond finesse.
Hope that helps.