The extended trot in ponyplay involves a long stride length while still maintaining the rhythm and balance of the trot. This is not really the opposite of collection, nor is it dropping all rein contact and allowing your pony to do what it wants.
To achieve the extended trot, start your ponygirl or ponyboy at a working trot. Take up some additional rein contact to ask for some collection. Use your crop or leg to maintain the impulsion of the gait since your pony may attempt to slow down or drop down to a walk.
Once your pony is on the bit at a stable, slightly collected trot, ask for additional impulsion with your crop or leg without asking for additional collection. Your pony should respond by translating that extra impulsion into an extended forward stride (versus a higher step as with the more collected gaits). However, your pony should remain on the bit and maintain proper head carriage.
Again, one of the more important things to teach your pony is to keep the rhythm of the trot and maintain balance. You want your pony to be moving in a controlled manner, but with a long stride.
A bio-equine (below) demonstrates the extended trot. Note how the bio-equine maintains an incredibly long stride length while maintaining a good degree of suspension.