I was thinking about this topic, and had a pretty good idea why it might work. So what I think happens is when you switch characters, the character you switched from gets put into their default position (their safehouse). But then what about when you switch back instantly. What I think happens is an npc spawns with your character's model and car. And your REAL character gets teleported to their safehouse. So when you (for example) save warp Michael to his safehouse you hold down the character switch button so it thinks you're switching to someone else, so it teleports Michael to his default position, but you can't switch while loading into the game, so when you load in, you stay with Michael, who gets warped to his safehouse. Also this is from me, someone who doesn't know a lot about the game, which means this might not be true. Let me know your ideas of why it works. : )

Also this is why car duping might be possible. it spawns the NPC with the car, but it can't delete your car because you're still inside of it.