In this exercise, you're dragging the sled behind you, but you're also pushing against the suspension trainer straps or ropes. The fact that you're holding on to two unstable handles forces the upper body and core muscles to work harder than they would with the stable, upright pole handles of the Prowler-style sled.
The purpose of this column is tri-fold. Purpose number one is as a "refresher course" for seasoned lifters. Sure, deep down, you may know this stuff, but it's been so long since you first read about it that you've forgotten the rationale behind the subject matter.