Darren

This is a very good question and many thanks to those who have already provided answers. 

In my opinion, when a player defends by letting you have another go they clearly are in reasonable command of their racket. However your response must be to test. This requires a combination of control for pace and control for placement. Your role is to create an opening and you do this from testing. If the defender is standing reasonably well forward then, as Matt said, punch in an attacking clear and see how well they move to rear court.

Aside from that you need to mess up the timing on their defensive shot through very good placement and pace. Adding slice can sometimes be the killer blow because this drops the shuttle shorter and makes your opponent dig the shuttle from near their feet. Testing various places on the body is also worth a go to see how well they handle a well-placed smash. It's also worth hitting a fast drop shot down the middle between both players to see how they respond.

At the end of the day the smash is not always about power. Placement is far more important. Placement with power is the most effective shot in the book after a good low serve.

Paul