Pulling trousers tight
Pulling the trousers tight is a common preparation when a corporal punishment is supposed to be applied on the seat of the pants instead of a bare bottom spanking. The trousers are pulled tight over the buttocks in preparation for and during the spanking. Normally the spanker grabs the spankee's pants on the belt loop or the waistband and tightens the pants by pulling it up towards the spankee's back.
There are multiple benefits of this preparation. Firstly the pants are stretched and the material becomes tighter, this will bring to light any "stuffing" issue either in the back pockets or under the pants. This also minimizes their protective effect, and all wrinkles of the trousers smoothed out will have an aesthetic effect for some spankers.
But there are further side effects when the trousers are pulled tight for a spanking. It lets the "target area" (the buttocks) become clearly visible and makes it easier to aim. And holding the spankee in place during the spanking is also easier when the spanker grabs the spankee on the waistband. And in the case of short pants it raises the hemline for leg smacking.
Another example showing the "pulling tight" method (spanking postcard, before 1918)