It depends on how your carb is setup. If you end up at some weird setting that makes it run lean coming out those turns (or someone makes a mess of setting the popoff and you’re stuck with it for some reason), you could try choking varying amounts of choke before the turn and see if it perks it up a little.
Or you could just richen your low jet 
and/or check you don’t have an air leak somewhere.
Try it (choking) out.
I was thinking at Badger, going into turn 1 on a KA, we brake, but not enough to get into the low jetting for the carb (I think) and so there’s a slight delay from throttle on to the engine picking up, and was wondering if choking it might help.
Also, mind games at the club would be very fun then
It won’t be faster. On a modern kart engine, if you’re struggling with jetting, you can just tweak the carb to fix it. Plus, if you choke the engine at the end of the front straight at Badger, you’re going to look like a wanker. 
Those mind games we play though 
Definitely don’t want to be a wanker, so let’s look at it from the other end. Line pinching: when to do it and why.
I have a tight section at my local, from a hair pin up a hill to a tight right- right combo. I sometimes instead of lifting for the first part of the right- right bit, I choke.
Its probably not doing anything for lap times but its a relatively slow part of our circuit and easier to do it there than into the faster corners (the Local doesn’t have any sweeping corners off straights that are easy to choke in).
I really only do it if i think the engine temp on my KA is getting too high. A cylinder head temp of 160’ is supposed to be the top safe temp, but I don’t like going over 150’
@Peter_Zambos Funny story, about line pinching
The KA isn’t supposed to need it either, but of course everyone does it. Pole sitter was pinching his line as we went through the tracks cut through. A rock bounced up hit his shoulder and trickled all the way down his arm and got stuck in the gap between the throttle lever and the bit that the cable comes out on the side of the carb, this jammed the throttle closed. Couldn’t figure out what was wrong, so we started the race with out him. Apparently the swearing was quite impressive.
It made for a good weekend, he was super quick anyway but he had to start from the rear for most of the event, it was a great game for me, to see how long I could hold him up. Not long usually, and showed his class in the way he cut through the field to either win or be second.
So in that case the answers are - “when” never through the cut through. and “why”- because something you would never expect to happen will!
I do it on the KA on the starts. Careful though, I killed it on the start at GoPro the first time I tried it this year.
I only do it because it’s easier than grabbing the needles. With the clutch on the Yamaha, it wouldn’t be necessary, nor would it on anything with any power. The KA it works because they load up a bit when starting.
On the Komets we used to have the carb turned in half a turn and just richen it up as we accelerated.
It somewhat depends on the engine, carb type and speed of rolling starts. Back in the FA days it was required skill because the polesitter would often run the grid down super slow to oil up some competitors.
I would suggest that any engine that on the rolling lap is running below its normal operating rpm range would need less fuel at that speed. How you achieve that, be it altering the jetting or pinching the fuel line doesn’t matter. Pinching the fuel line is a little easier to do, little harder to manage.
One caveat, doesn’t work with float carbs because you can’t judge how much fuel is in the float chamber.