IMG requires that you pay 20% percent of the first $5,000 in medical expenses (for visitors to the US & Canada only) after the deductible (same as "excess") is met.
For example: You have a serious accident in the United States that requires hospitalization for a week. The hospital bill is $10,000. If you have a $100 deductible then you would pay $100 (leaving $9,900 to be paid) and then you would have to pay 20% of the next $5,000 which = $1,000 (max out-of-pocket with $100 deductible =$1100). IMG would pay the remaining $8,900.