When you eat, depending on the glycemic index of the food you're consuming your blood sugar starts immediately going up and can quickly peak at dangerous numbers if the food is sugary.

A diabetic will want to take dose of insulin a bit before eating in order to send their blood glucose level on a lowering trend. If you dose it right the two waves semi cancel each other out and your blood sugar goes up some, but hopefully not a huge amount.

If you get the dose wrong, it drops dangerously or rockets up and you have take correction doses.