Captain Seafort wrote:Why? Ren was suffering from emotional turmoil and the after-effects of a direct hit from a bowcaster (albeit mitigated by his armour). He was also being cocky and underestimated both Finn and Rey. None of these circumstances are likely to be repeatable.
There's a ton to pull apart here.
Let's start first with the bowcaster hit. We are shown
repeatedly what happens when you get hit with a bowcaster directly.

What happened to Ren was a glancing side hit. We can see this because a) it's where he's pounding on the wound b) the blood is dripping on his side, not down his front c) if you actually watch it again it
looks like he got hit in the side.
Lastly if he'd been hit center mass he'd have gone flying like a rag doll. He did not go flying, ergo he didn't receive as much energy as the targets we see hit directly. Either that or he's significantly heavier than he looks.
Let's not forget that despite that wound, he was able to not only catch up to Finn and Rey, but
get out ahead of them to cut them off. Hardly the kind of thing someone suffering a serious wound would be able to do.
But if we're going to discuss physical condition going into the fight, let's not forget Rey.

That to me looks like a pretty nasty impact. Assuming the force push doesn't hurt her at all, she's hitting a tree pretty damn hard there and taking a pretty decent fall, which she seems to land pretty heavily. If she had broken ribs or fractured limbs I would be wholly unsurprisied. She doesn't display any of these symptoms, but then Ren doesn't display any handicap from his bowcaster hit either. What she
does display is being knocked out and blurred vision, which by any estimation is a concussion. Being concussed like that can affect pretty much everything, hearing, vision, balance, reflexes and most certainly- which may be relevant for a force user- your ability to focus. Given it knocked her out and blurred her vision rather than just dazed her it's got to be a reasonably serious concussion at that.
As to emotional turmoil, yes, I wholly concede that Ren was suffering that. But stop for a minute and think about Rey, because the last few days haven't exactly been awesome for her. She's been forced to abandon a place she's strongly against leaving- under fire from TIE fighters and stormtroopers no less. That experience alone would be terrifying. But then she goes on to face the monsters on Han's ship and nearly lose Finn. She then has to experience combat again, against
stormtroopers no less, and ends up facing Ren. She gets toyed with, hunted down and finally has her bodily integrity compromised, followed by her mind invaded. And of course captured by the First Order- terrifying in anyone's book.
Have we established her emotional state may not be all that good right now? Because after that she's interrogated, mentally invaded again and when she gets free she has to creep around a base for a few hours, one accidental noise away from capture or being blasted. If I were her, I'd be freaking out right now. But it doesn't stop there. Next she gets to watch her father figure/mentor cut down in front of her. Then she gets smashed into a tree and wakes up just in time to see her only friend in the world Finn cop what would certainly appear to be a fatal blow. And of course maybe she's just a *tiny* bit scared of Ren. I mean, last time she fought him he basically locked up her body with TK and then knocked her out by waving his hand at her. She must be super confident going into that fight. So yeah, Kylo Ren is emotionally unstable but Rey can't possibly be in that great a shape herself.
As to the underestimating, that's odd because he outright says she's strong in the force and he's already lost to her twice that this point. He didn't go in thinking he was better and it turned out she was, he actually
was better. He fights Rey mostly 1 handed, pushing her back again and again until she hits the precipice. He is 100% in control of the fight. Then, in what felt to me like someone reminding an RPG player 'Oh you have this ability' Rey hits the I win button and the whole thing is reversed.

She realises 'Oh fuck I can just use the Force!' and boom! Instantly she goes from retreating and being pushed back to whooping Ren's ass. This is the third time we've seen her draw on the Force, opposed by Ren
and Ren loses. He loses the mind probe, he loses the force pull on the saber and now he's losing this battle. The implication is clear- Rey is stronger in the force than he is. Not just stronger but more capable. Shit, just compare her first saber pull to Luke's. She's insanely strong. It's like she just gets +5 to all stats or something because a moment ago she was running for her life and now she's in total control.
And that's a problem because the next time they meet, she'll be trained and she'll start off
at least that powerful. That's the benchmark Ren has to meet
just to stand a chance. Because once she used the Force, it wasn't a fight. It's not even close. He gets off two attacks, one of which is a flailing defensive swipe that comes nowhere near her. The rest is just a barrage from her that ends up with him disarmed, wounded and lying on his ass. He's saved only by the fissure.
TLDR: The reversal where Rey goes from losing badly to winning massively implies her completely untrained force powers are insanely strong.
Sorry for the long post.