Hi Blueskies, Yeh, I guess the terms can be used interchangeably, but to me fishtailing was when any kind of vehicle, trailer or powered starts to swerve from side to side. Yeh that'll deffinately do ya in. My term porpoising is when the trailer starts to get a rythem where it doesn't have enough weight on the ball and the heavier back end lifts the front end and the trailer hitch is jerked up against the ball hitch on the back of your tow vehicle. As soon as it gets alittle movement up, providing that the hitch is locked on, forward momentum of the tow vehicle jerks it back down and it bottoms again on the ball. the the weight in the back causes the hitch to try to jump off your ball again and the thing just keeps doing that continuously in a rather quick rythem. It literally can tear the hitch right off your vehicle and if it doesn't it'll keep jarring your vehicle so much you'd wish it did. With just alittle weight on the front the hitch will always want to stay down. Dave.