When you activate its ACT effect paying the Counter Blast, it gains +5000 Power and loses Twin Drive until end of turn, and in addition it gains the effect in quotes until end of turn.
Question: When this unit attacks while it is on (VC), can I still Drive Check? Answer: Yes, you Drive Check each time this unit attacks if that is the case. This unit only loses Twin Drive.
Question: If I have already activated its ACT effect, can I still Twin Drive if I do not want to attack a rear-guard? Answer: No. It loses Twin Drive immediately until end of turn, that is not part of effect in quotes of "When this unit's attack hits a rear-guard".