It's as simple as a filter. You and your intended summoner put the same password, say, "boner", and you'll filter through any matchmaking to only reach your pal. It's good because it supposedly scales down/up if there's a huge SL difference. It's bad because it's waaaay too bare and gamey, encourages unavoidable gangrape to invaders, and makes it a tad too simple IMO. Besides, it was from Bloodborne anyway.