In the attached diagram there are two items. The first is a sail that is over trimmed and stalling, indicated by the backside telltale dancing or acting up. They say it develops 100% efficiency -- or the same effect as a barndoor.
The second item is a sail trimmed properly as indicated by the backside telltale flowing. It is said to develop 180% efficiency. Obviously much better in the power department.
The funny thing is that when the backside telltale begins to flow, the jump from 100% to 180% is immediate.., and not in steps or in graduation. So, you get an immediated 80% more power, so to speak.
Now, when you are sailing downwind with the spinnaker your mainsail is normally sheeted in as if it were sailing a close to beam reach.
After taking down the spinnaker, the boat slows down considerably and most folks don't adjust the main by letting it out. Therefore it stalls -- dropping the main's efficiency to 100% -- you know, the barn door again.
Next, most folks round the mark with this configuration and when the mainsail goes from stall to full power, it is immediate and not gradual. Therefore, many boats flip right there at the leeward mark. That is why you see so much carnage at the C mark.

The answer is that after taking down the spinnaker, ease the mainsheet so the sail is not stalled. Then as you are rounding the mark sheet and steer at the same time, keeping the backside telltale flowing the entire time -- you will not experience the sudden rush of power that way and will be sailing through the turn faster as well.
Hope this helps,
Rick