[He screwed up his face.] Oh, how to explain the mechanics of the infinite temporal flux...

[He settled for a small nod to start with.] Well... yes and no. You see, time isn't exactly a straight line, it's more like a. Wibbly wobbly timey wimey ball of stuff. I should get that copyrighted, really... It's a mess, basically. So once I piloted the TARDIS - rather daringly, I might add - and scooped you up out of an impending explosion, the thing that I was trying to prevent - namely, you dying - never even happened in the first place.