As I said: you search for something, then select it.
You clear the search filter, and all your selections are kept.
Now you search for something different, and select something else.
You clear the search filter again, and all your selections from before are kept as well, meaning you have now both selections from the first search and the second one.
So now, when you start entering something in the filter box, the selections are stored. Then with the active filter, you select something and that selection is added to the selection that was there before you started filtering.
The selection process itself hasn't changed: you still need to use Ctrl+Click to do multi-selections. And while a filter is active and no filter is active, a simple click will select one item and deselect the others. But when changing filter/no-filter, the selections are kept.
Stefan