For securing a wheelchair in a car, I have seen tie down systems and docking systems. It can be a bit confusing, because Q'Straint makes both types.
The EZ Lock system I use is a docking system. With that system, there is a docking base fixed to the floor of a car and a plate with a protruding bolt attached to the wheelchair. That bolt engages with the docking base, which locks the wheelchair in place.
Q'Straint makes a very similar docking system (and Permobil used to make one). The Q'Straint docking systems are the QLK-110 and QLK-150.
Q'Straint also make tie down systems, such as the QRT Standard and QRT Deluxe. These systems have retractable straps anchored to the floor of the vehicle. The straps must be manually pulled out and connected to appropriate points on the wheelchair. I have used these systems extensively on busses and they work very well. Unfortunately, they require effort from someone other than me to secure me and the wheelchair into the vehicle.
Note that in addition to securing the wheelchair you must also have a way to secure yourself. I have a seat belt system made by Q'straint (mine looks like the Q8-6325-A). I rarely use it, as I am still able to transfer to a regular seat and use the seat belt there.
Now, with all that background, here is what I think Karen was referring to. When a docking system is used, the wheelchair has a plate attached to its bottom with a large bolt protruding from it. This bolt is often positioned close to in line with the drive wheels, so if going over an obstacle, such as a threshold, the the drive wheels lift the wheelchair up and the bolt is also lifted. This is what happens with my Permobil C500 wheelchairs. However, on the Permobil F5, the EZ Lock and Q'Straint (QLK) bolts are place a bit back from the drive wheels. So, when the drive wheels drive up over an obstacle, the bolt is not lifted as far up. Thus, the bolt catches on things.
This is quite annoying, especially when driving on uneven pavement. On my wheelchairs, when the bolt drags on high pavement, it sounds a bit like fingernails on a chalkboard.
I have been told that on the F5 locking plate, the EZ Lock bolt is mounted further back than the Q'Straint bolt. I have not personally confirmed this.
The F5 has less ground clearance than the C500. I find that the C500s easily clear the EZ Lock plate in my van, but that the F5 drags a bit on it.
With the EZ Lock system, the bolt that engages with the docking base is actually screwed into the wheelchair's plate from the bottom and is secured in place by a locking nut. I presume a similar arrangement is used in the Q'straint QLK systems. It is pretty simple to loosen that nut and remove the bolt, or to replace it with a shorter one. I have spent a bit of time getting the shortest possible bolt that will still lock the wheelchair into the EZ Lock base. Still, that bolt catches on many things when I travel over rough terrain. When we know we are going to be traversing rough terrain my wife removes the bolt, reinstalling it when we return to the van. I keep an 8 inch adjustable wrench in the van door's pocket just for this purpose.
I feel fortunate to have an interest in many things, including electrical systems, mechanical systems and software. Having a 30+ year career in a more challenging area of high tech with lots of software development, algorithm development, and mathematical explorations also helped. But, the greatest advantage I have is a set of friends who are highly capable and very willing to help me take on just about any challenge.
I blame most of it on my dad, who was an abstract mathematician, Air Force pilot, electrical engineer, and all around good guy!