Adding weight to sled versus tying it down to a structure underneath, there is surely a difference. I don't own one, but I did use one a few times in the past.
You don't tie down the rifle to restraint its muzzle raise, that's given. The rifle rotates up as if fired from the shoulder. Why use the sled then? It is to reduce the rearward recoil force.
I won't go into the physics details. It is the principle of conservation of momentum. You need to increase the mass of whatever behind the rifle. Leaning into the stock with your shoulder is one way. A sled with lead weight is another.
Tying the sled down to a table doesn't really increase the mass. The sled still slides on the table top, unless the tie down force is so strong that the sled is practically part of the table, in which case the sled takes up the mass of the table. For that perhaps you will do better tying it "down and forward". You may notice tying it down helps. That is because of the braking action of the frictional force between the sled and the table. A sled with weight is like a truck being by a small car. A tied down sled is like a van with a set brake being hit by a small car. A tied down and forward sled is like a van with set brake and its rear end butting against a truck being hit by a small car. Three different scenarios.
Still be careful if the table is fixed or very heavy. It is equivalent to setting the rifle's stock against a wall. Things may start to break. At least the rifle muzzle may jump up higher and higher till it rotate back and over, like in the video.
Bottom line. If it works, it works. If it doesn't, you will find out sooner or later.
-TL
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