Blood Angels hunger for close quarter fighting more than any Chapter. Often a Blood Angels leader will entreat the Tech-Marines to improve the engines of a Rhino or Razorback to get as maximum speed out of it as possible. A vehicle with an over-charged engine can attempt to go faster, although there is a risk of burning out the engines completely!
At the end of its move, a vehicle with an over-charged engine can try to go an extra 6". It must first take a Difficult Terrain test and if successful can be moved forward up to 6". Blood Angels are adept at these fast assaults and can still disembark from a Rhino or Razorback that has travelled more than 12". A vehicle that uses its over-charged engines may not shoot in the same turn.
Errata — Chapter Approved: Codex: Blood Angels Q&A (v1.0): The 6" over-charged engine move must be straight ahead — it is not a normal move that can go in any direction.
A dozer blade lets a vehicle re-roll this Difficult Terrain test if it will not move more than 6" that turn, but by the rules it doesn't stop the engine itself malfunctioning. As a house rule, no reasonable opponent should object to letting a dozer blade stop the over-charged engine test from misfiring too — but this rarely comes up anyway, since a dozer blade is not useful when a vehicle moves more than 6".
This 12"+ disembark exception does not extend to force marching (the White Dwarf rule): if Blood Angels in a Rhino force march, they cannot disembark from it at the end of that move.