As the piston moves down, valves open on the air filter side allowing air to fill the piston. The engine is basically a big air pump. Air has to have somewhere to go for it to flow. Damage to the engine would be highly unlikely. If the exhaust system won’t allow any air out, the engine won’t run as it won’t be able to draw air and fuel into the engine. Air is drawn into the engine through the air filter and the fueling system by the movement of the pistons. The engine would stop if someone blocked it successfully because it can't expel the exhaust gas on the second stroke of the piston. As the piston move back up after compression and ignition, the valves on the exhaust side open allowing the air to be pumped to the exhaust system.