Can certain vehicles be started by keeping the gears engaged?

Yes, a manual transmission vehicle can be started by turning on the ignition, engaging the clutch, pushing the vehicle and letting out the clutch when enough speed is reached. When the kick start broke on my Honda when I was a kid I did that for months before I had the money to repair it, I found that being in a higher gear made the start easier as long as you are quick to reengage the clutch when the motor catches.