DISCLAIMER: I cannot stress enough that what works for me may not be what works best for everyone. This is a “take it or leave it” thing!
First, using the handbrake does not necessarily make the car stick to the trailer like glue but it does help reduce some movement. The car can still slide around every which way with hard jerks, braking, or on bumpy roads.
Second, the main reason to put the rear straps straight instead of crossed is that this method is the best for preventing forward movement under very hard braking or in a collision. Braking always exerts more force than acceleration. When crossed, the straps do double duty, as they prevent most forward movement and side to side movement but they do not eliminate them entirely. Think of it as simple leverage rules and how you’d position yourself to hold something from moving. On a trailer, the perpendicular axis is best for side to side control and longitudinal is best for front to back control.
Third, the weakest strength of any through the wheels straps (no matter if crossed or not) is actually upwards movement. If you’d turn the trailer upside down with the car on it, the car would most likely hang in the air a few inches to a whole feet. Think about this when planning for the “worst that can happen”, aka a very bad accident. Other people often forget to check their mirrors to see if the vehicle that just passed them is towing something, especially at night, or if you passed them on the right (which is dumb I know but it happens)…
Fourth, the advantage of through the wheels straps are that the car rides on its own suspension, reducing shock transfer to the whole car when you take on nasty bumps. By using frame attachment points instead to hold the car down you compress the suspension and the opposite occurs. Also, if you have a softer suspension on the car and ride on nasty bumps, it can actually compress the suspension more than the tension you put with your straps. That makes your straps loosen for a fraction of a second and it actually can make the car move around a little bit on the trailer. Don’t take this seriously and you may potentially end up with the rear wheels of the car at the edge of the trailer on one side, making unloading it a serious problem.
Fifth, there’s also over the wheels nets (those who attach front and back of the wheels) that work well in most situations but attaching them to a car hauler trailer can be a challenge, especially with low riding cars (such as most tracked STis). Wheel nets with side attachment points are great to control side to side movement but suck big time in controlling forward or backward motion. However, wheel nets are the surest way that the car will remain on the trailer even if you’d flip over in a deep ditch. They also let the car ride on its suspension so that’s a good thing.
Lastly, all of the different systems have their strong and weak points. All of them work fine… until you get into real trouble! My end all solution is to use 8 attachment points; 4 side attachment wheel nets (best side to side and upwards control) and 4 through the wheel straps attached as straight as possible (best front and back motion control). That way, this ensures that your car will NOT move even if your trailer would be put in a gigantic gyro ball and rotated 360 degrees in any direction.
This comes from my 15+ years experience in trucking, as load shifts can be very costly in terms of customer claims and possible criminal lawsuits. Remember, a guy with a pickup truck and a trailer will probably get away easy if he causes an accident due to a bad load secure. A trucker will make time in prison (seriously) so a competent one will know his stuff and always prepare for the worst. What works well in normal conditions might fail in very bad ones. A crossed “X” setup front and back is fairly safe and a time saver but remember that how you tie your car down is how you’re willing to “risk it”! Oh, and do stop once and again to check on your load and do tighten any loose strap!
Low budget prevented me from acquiring the exact straps I wanted but my overall setup looks like this:
The car never moves…