Can't find any real use to it.

Acts similarly to Save Warping on Architect's Plans. You can achieve similar effect by simply killing yourself or entering another mission while Michael talks to Franklin on the phone. You'll be teleported to the character's house after you finish or fail the mission.

Ending Choice: game won't let you finish the taxi-mission no matter what. You can't pickup any mission markers while Franklin talks on his phone, you will softlock or die if you try to start parachuting mission (the one with helicopter, since you can't press 'e' while on the phone to start normal one).

1. Press and hold 'e' button (accept the race) near one of the Franklin's races.
2. While holding 'e', press and hold the button to change your character. Alt, f5-f7, whatever.
3. Here's the tricky part. You have to release both buttons almost at the same time. I can't tell the exact timings but it seems you have to release 'e' button few frames earlier than your other button.
4. If everything is done right, one of this things will happen:

• If your second character has no vehicle and IS NOT in the switch-cutscene, Franklin's vehicle will be duped and you'll start a race in this duped car.
• If your second character has no vehicle and IS in the switch-cutscene the race will glitch. The race will begin but you won't be teleported to the start line, instead some random NPC will take your place inside Franklin's car and will drive around in it. (for whatever reason xD)
• If your second character has a vehicle, you will start a race in his vehicle. In my video I purposely spawned a buzzard to show this behaviour.

I can't find a thread where people can post some random glitches that might be used somehow in speedrunning of this game. So, yeah, I'll create one.

Let's start with this:

You can find a link with an explanation in video's description.

I don't think FI cars are usable in any way in a speedrun, so the interesting part here might be transfering mission states from one character to another + unfreeze mechanic when leaving the taxi before the taxi mission actually starts. For example, you can unfreeze yourself while buying stuff in food stores, picking air\water-vehicles, etc.

Same way you can transfer Franklin's races to other characters. I don't have a video for this one but I'll record it when I have time.

You can find few other glitches and their uses on my channel but to me they seem even more useless than the one's above.

Anyway, maybe someone will post something else here.