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.
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Tip: The Back Burner
Finish off your back workout with a few rounds of this. Bonus: It'll wake up your rhomboids too, which will help improve your crappy posture.
Cardio Progressions
The last time it happened I was getting changed in the locker room at my gym. Two pudgy-bellied gym members (PBGMs) were discussing the relative virtues and drawbacks of doing cardio as part of their training program.
Tip: The Best Chest Exercise Since 1911
For complete chest development, don't forget this classic. Bonus: There's a new way to do it for even better results. Check this out.