As for now, here is a description of how people are exploiting this glitch:
* Have a class with the following perks (Marathon/Lightweight/Whatever)
* Get a 4 kill-streak to obtain a Supply Drop (also works with Sentry Gun Drop)
* Equip the Supply Drop smoke canister
* Instead of throwing the smoke canister, you keep it equipped
* You are now able to run forever at a much much much faster pace
There you have it, this is how people have been running insanely fast in Modern Warfare 2. It is most popular in objective-based variants such as Capture The Flag.
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