There are some features of MaxCare that start right away, such as free loaners during warranty repairs. Non-powertrain and roadside coverage starts when the original 3/36 expires... that takes care of your electric/electronic components, and mechanicals that are not part of the powertrain (e.g. HVAC), as well as roadside assistance (out of fuel, flat tire, etc.). If your powertrain expires at 5 years but with less mileage than the MaxCare plan, then MaxCare will cover the powertrain for the remainder. If your powertrain coverage ends because you hit 100K before the 5 years, then your MaxCare will have already expired as well.
One thing I have recommended in the past - if you are getting a service contract, make sure it will outlast any factory coverage. With the 3/36 base warranty and 5/100 powertrain warranty, you don't want less than 5 years and 100,000 miles of service contract coverage. With my 2005 Ram, it had 3/36 base and 7/70 powertrain, so I got a 7/100 service contract.