Different kinds of "two direction" finesses in bridge?
The simple, unhelpful reason/rationale is that it gives you the best chances under the circumstances.

For instance

Kxx

AJ9

You need 3 tricks and know that LHO has the Q. Your best shot is to play RHO for the Ten, (which is ~50% assuming no other information is given). Playing the J first sets up the finesse position for the Ten. (Consider a similar one way situation, xxx opposite AJ9. Needing two tricks, you first play to the 9)

You can try playing the AK to drop the singleton Q or doubleton QT, but that is inferior in terms of chances.

I remember reading a book where in one of the problem hands, the right play in trumps was to  finesse one way, and if that wins, repeat the finesse the other way! (I might add the hand later if I remember it).

Not sure if that helped.