After finishing the game I took blocking posts on the workshop. So what is all that magic behind them? Let's see...
Situation one - we park a truck to block the barrier. Due to "bubble physics" the barrier stays open as long as we are don't come close with any other vehicle. The barrier animation is just an "illusion" well... animation, as it is not the real thing stopping us from crossing. It can collide with the car model but it does not cause any damage. We can park a log loaded truck nearby but as soon as we switch to log free truck the barrier opens and we can pass.
What we can see here is that if we are in a car not touching any wood the barrier animation remains open and we can cross - truck no1 and the truck off the screenshot. Truck no3 cannot pass regardless the state of the barrier as it is just visual interpretation.
You also cannot haul/winch that truck through the post by any other truck as the winch point keeps breaking.
An interesting thing happens in circle number 2. You cannot pass the logs directly over the barrier as the physical blockade ("magical barrier") is being implemented there, but... if you take your truck and park it near the fence, the "magical barrier" does not extend there and you can pass the logs over the concrete fence without any problems.
Last but not least, you cannot push a truck through the post (but I need to retest that). The blocking post works by detecting that the car is in contact with logs. Logs don't have to be in any specific place, they can be in the loading zone, in the grabber/crane, on the roof or even on the road. If you look at the logs in the red circle, what you can see is that they are not loaded, nevertheless if we want to cross the barrier and our wheel(or any other part of the car) is in contact with any log it closes the "magical barrier".
I also tried grabbing some logs lying on the tarmac being on the other side of the barrier. As long as you do not have any contact with logs the animated barrier and the "magical barrier" remain open. But as soon as the grab touches the wood your truck is being pushed a few meters back by "magical force field"
One more, you cannot damage the log loaded truck by hitting the barrier as the truck is being slowed down to a complete stop on contact with it.
TL;DR Blocking post work by detecting any physical contact of logs with any part of the car. If that happens in the place of the barrier a physical invisible blockade is being deployed. The barrier opening and closing is just an animation and you cannot damage your truck by hitting it as it slows it down to a compete stop.