“Waving the bloody stump” means you have a critical wound. You’re not allowed to lie about it. If you don’t have a critical wound but wish to surrender you can cut your engine to low throttle and glide at 40 MPH. There is no rule that doesn’t allow you to turn your engine back on and fly off, but after you do that just once the player you deprived of a kill is apt to be a bit bloody-minded to you in the future. That’s why some groups play with a house rule that you can’t turn your engine back on (and why it’s a rule often used in tournaments).