whoever told you that, knows noting (well maybe they might know a little) about engines. 4-stroke since they have valves and cams if you run (or should I say try to run) a 4-stroke backwards it will suck air in the exhaust valve, but you can't run a 4-stroke backwards for that reason.

think about it. if a engine sucked air in through the exhuast then how will it get fuel to run? and second, how will the fuel and air mix and get forced into the sleeve? whoever told you that go up and ask him/her if an engine will run off sand, and see if they say yes. because thats basically what there saying. hopefully that will clear some thing up.