Okay, so shortly before Moiraine tackles Lanfear through the twisted stone doorway, e was sneaking up on Lanfear. But right before she piledrivers into the Forsaken, she e!braces saidar. 

 

why??? So she was trying to sneak attack Lanfear, her entire gambit rested on her not being noticed and incinerated by the irate Forsaken. Embracing the source would have immediately drawn attention to her. 

 

I have some theories about this. 

 

1) She wanted to be noticed to distract Lanfear from killing Rand. But wasn't that the entire point of attacking Lanfear bodily in the first place? Why risk her tackle for a momentary distraction?

 

2) She wove something to give her the upper hand. If she did, it wasn't a shield, because with the amount of saidar in Lanfear at the time, I think it would have taken a full circle to do so.

 

My last theory remains, to me, the most plausible. 3) Moiraine embraced the source to melt the doorway as soon as she and Lanfear were through. How on earth she did that, I don't know, but she did rediscover balefire, so destroying the doorway wouldnt be impossible. Plus, she probably got the bracelet angreal when she did do it.

I assumed it was to get Lanfear's attention.  The biggest part was to get in the wagon unnoticed.  Notice she didn't do it until she was close, once she embraced the source it was too late for Lanfear to stop it.  it also made Lanfear think an attack was going to come from the power, not Moiriane tackling her.