Doing that will shift the transmission into the lowest effective gear when you hit the accelerator, improving your acceleration, skipping gears if necessary.

So for example, you come into the turn at about 75 in 4th gear. You hit the brakes into the turn and you hold the stick to the left. 40% into the turn, the vehicle is now at 35, you are ready to accelerate out of the turn, you apply throttle, the transmission will shift to 2nd and let you take off.

Most standard sequential gear boxes like the regular Chrysler one will just shift one gear down, so to shift to second from fourth, you'll have to pump the stick twice to the left and unless you know your shift gates well, you may still not pick the best gear for the RPM your engine is in.