Selective pinning allow the elite stand to choose in wich order they will fight.
If the elite stand is surrounded by 4 enemy stands, he will fight the 4 stands but will choose wich opponent it will fight first, then next stand, etc...until it dies or until victory.
Example:
A close combar terminator stand (CAF +8 & elite) is engaged in CC by 1 ogryn stand (CAF +6) and 3 IG tactical stands (CAF +0).
As the IG troops are outnumbering the SM stand, they normally could decide wich stand will start fighting first. But as the terminator stand is Elite, SM player decides to fight first the ogryn stand then tactical stands. If the termi stand was not Elite, IG player should have choose to play 1 or 2 rounds of CC with standard IG troop than with the cc bonus dice(s) play the ogryn stand.
Hope this help

Edit: I think the Elite rule should be globally reworded. The tiny resume is not clear enough to be easily understooden. Maybe at one example could be added. We aleady discussed this point, maybe it's already done
