It doesn't. The engine uses petrol any time it is running, but
not more when braking.
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Depends on the bike and it's gearing. At around 30mph, i will
downshift from 4th to 3rd for engine braking. At around 18mph, I
will downshift from 3rd to second for engine braking. I will not
engine brake into first gear.
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No, it would not be able to handle engine braking.