The rule of restricted choice works says this.

 

The play of a card which may have been selected as a choice of equal plays increases the chance that the player started with a holding in which their choice was restricted.

 

If a player holds a singleton Q or singleton J their choice is restricted.

 

If a player holds the doubleton QJ the cards are equal so they have an unrestricted choice about what to play.

 

Because the LHO played a card which may have been choice of equal plays then there is a higher chance that their choice was restricted - ie the chance is higher that they did hold a singleton queen or jack.

 

The simple rule of thumb is if you see LHO drop an honor in a suit holding like this (there are many similar ones) then the odds favor finessing RHO.

 

 

For the mathematically inclined here's the reason...

 

One player will hold QJ doubleton 6.78% of the time - so the chance that the LHO has QJ doubleton is 3.39% - and they will randomly play the Q or J from this holding. (They can play either - their choice is not 'restricted')

 

The probability that LHO had a singleton honor is 6.22%. Here the choice of card LHO can play is 'restricted'.

 

So the chance LHO had a singleton honor is 6.22% and a doubleton honor 3.39% (ie odds favor finessing rho by alomost 2:1)