As a2zmom said: It's as much a suicide leap as a heroic gesture.

I guess the whole 'summers' blood' thing doesn't quite make much sense. The portal needed the blood of Dawn to close and in Blood Ties, Buffy had pointed out that Dawn's blood was just like her own. So basically, their blood is alike, ergo - Buffy could close the portal just as much as Dawn could. That doesn't quite explain why it makes sense, though...