Can someone plz explain to me about sliding tandems

It is for some/many. And I'll take this opportunity to caution you against considering the 250 lbs/hole a "hard and fast rule". It's a rough rule of thumb at best but has gotten lots of drivers in trouble who made that assumption. Also, in most of my situations , 250 lbs is too light by quite a bit.

 

Remember to move your wheels to weight. Watch you don’t exceed 41 feet from kingpin or bridge law

 

True but it's much more than hole spacing. More than that, it's about product weight and it's distribution [nose to back] and how far back the [heavier] product is placed inside the trailer. Every load is different especially multi-product loads and multi-shipper and multi-stop loads.

 

Correct.

Specifically, for any given truckload the weight shift per slider hole is determined exactly by the following three things:

(1) The balance point, aka center of gravity, of the loaded box (trailer minus axle assembly). This point depends heavily (no pun intended) on how the shipper loads the freight.

(2) The hole spacing.

(3) Which two holes you are moving from and to; less weight gets shifted the closer the holes are to the rear of the trailer.

According to physics no two pairs of adjacent holes will produce the same weight shift for a given truckload. The shift is always less for the pair closest to the rear—it might not be less by enough (20 pounds) for a CAT scale ticket to report, but it will be less.

A hypothetical example that goes into more detail can be found here:

http://www.freightseesaw.com/axle_weights.pdf