Because first gear is quite a bit shorter than the rest of your gears. The difference in ratio can make it quite hard to get down from 2nd into first. Doing it a lot wears out the syncro. So I hear anyway.
The reason to get into first from second is if you hit a 1st gear corner, and need the extra power on the exit. Come into the corner hard under brakes, once your speed drop below the redline of 1st gear, swap out for first and take the corner in first gear.
Double clutching makes it way easier to get into first from second, or down any gear when you're rolling, especially when you're at higher revs. My car will not go into first AT ALL when above 5000rpm unless I double clutch.
Heel-toe'ing is just double clutching while half your right foot is on the brake in order to slow down quickly.




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