Impact carbs are carbohydrates that have a high impact on your blood sugar level. They are also known as high GI carbs (high glycemic index carbs) and are digested rapidly, quickly elevating blood glucose levels . Unless they are immediately used for physical exercise, the excess carbs will be stored as fat – drive by the body’s hormonal response . Refined carbohydrates and sweet foods are naturally higher on the GI scale. Avoid impact carbs unless you want to drop out of ketosis.
Non-impact carbohydrates are low GI carbs (low glycemic index carbs) that are digested at a much slower rate. They don't cause hormonal (insulin) spikes but provide more sustained energy. Vegetables that are packed with fiber naturally are digested slower which makes them a low GI food. On keto, the majority of your carbs should be non-impact.