Birth by Sleep Terra, Xehnort!? o.O - Spoilers-

Ok, how on earth did master Xenhort get inside terra when he had his armor on!? I mean at the point of impact, terra hit his armor, and that stuff goes on really fast!

and what was with the secret ending!? Terra just shows up out of nowhere with Ven!

someone help me!

 

Clearly Master Xehanort's darkness was significantly stronger than Terra's armour. The armour protects against the darkness of the Lanes-Between, but it isn't strong enough to block a concentrated assault of darkness. Terra would have been significantly better off dodging.

The secret ending with the Keyblades wasn't Ven and Terra. It represented the bonds between their hearts coming to help Aqua in a time of need. In other words, friendship did it.

Well they're more educated guesses than actual fact. It's just the way I interpreted the scenes. It helps to read over the journal entry afterwards, as it gives some details that might be missed.
I guess Terra got it wrong. Armour protected him from the darkness of the Lanes-Between, so he assumed it would also protect him from a darkness-engulfed heart being flung at him. Turns out it didn't.

 

Well, think about it, would you put your armor on when it's based to protect you from darkness, you instinctively would.

Plus, Terra may have not known it but his Lingering Sentiment saved what there was of him and

Spoiler

Since Xehanort's Heartless and Nobody have been defeated, Xehanort would return in Terra's body, thus bringing back Terra's heart and the Lingering Sentiment would probably be a key part in regaining Terra since Mickey and Yen Sid somehow discovered where Aqua and Ven are.

 

It was the darkness inside Terra (like seen as a weakness) that Xehanort grabs hold of and took over.

Spoiler

I'm assuming when he stuck the keyblade into himself his heart came out, in other words his soul which has no physical form meaning it can go through armor. Terra's reaction was just an impulse, like he thought it would offer some protection, I mean, what else could he do in that situation? Also it was key in the story for the armor to come around, otherwise you couldn't fight as the lingering sentiment

Also regarding the secret ending,

Spoiler

I think it was just to symbolise that Terra and Ven are waiting for Sora. Ven was asleep when Terra walked up to him which is his current state in KH2.

I wasn't sure what you meant with the secret ending but this is what I thought you meant.

 

So, if Terra hadn't of activated his armour at that time, there would be no Lingering Sentiment? 0_o

 

Nah, without his armor, there'd be no Lingering Will. Crazy, huh?

 

Yeah... It's weird to think that if Terra hadn't have done that at that exact moment, the whole timeline would have changed...