| Insulin is like a 'gateway' - it allows your muscles and liver to extract glucose from your blood and either use or store it. When you exercise, your muscles need more glucose so as to function, so a larger volume of glucose from the blood passes through the gateway created by the same amount of insulin.
Think of insulin like a door and your muscles as big vacuum attached to the door. When you take more insulin, you make a bigger 'door', causing more glucose to pass through into your muscles and liver. When you exercise, the 'door' stays the same size but there's a much stronger pull from the vacuum, forcing more glucose to pass through. | 
