Rubbing a cork on from the inside, what's it for?

[kmx321]

I've seen videos of techs rubbing a cork on the chip from the inside of the car, what's the reason for it?

[screenman]

It is supposed to help the resin flow, I have tried it on numerous occasions and found it not doing very much, I much prefer flexing the break from the outside. Also if you know how glass breaks it helps you understand why flexing from the outside is the way to go. I may be wasting my time but here goes, get a thick piece of card and cut a groove halfway through it, that is your glass with a subsurface crack/leg (which most are) now flex that piece of card and which way opens the cut wider? simples. Now if it is a surface break, it would be the opposite, so surface break which you will hear your scribe click on as you slide it over, flex from inside, subsurface no click, flex from outside.

I focus a lot on little things like this when training guys as I find it helps the message sink in better.