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.
Over the last few years it seems that just about every "in the know" performance coach has discussed the conjugate method and why they find it to be the most effective way to train an athlete. By now I think we all know the definition of conjugate or concurrent periodization and how it involves simultaneously trying to raise multiple qualities.