What is a drop back week?
This is a week where you reduce overall volume from your typical training week. But still keep some intensity!
When do you do it?
Before a training race (like this weekend!) or less important race. Or within a training block, you might cycle through 3 weeks build and then one week recovery or drop back, for example.
Why do we do it?
It allows recovery after significant overload or build up of training strain. Giving your body time to adapt to all the gains made in the previous few weeks. It can also prevent overtraining syndrome where you might be inclined to do more, more, more which puts you at greater risk of injury. Allowing yourself weeks to get more rest, sleep and catch up with friends is essential to longevity in the sport. Plus keeping yourself sane 
Wait, shouldn’t you be tapering for a race?
Well yes they can be similar in some ways. Decreasing volume but keeping some intensity pre race is very similar to a taper. However for some races that are not your key A race of the year you may be keeping some volume in. Plus if there is no travel time to consider for a local race you may have more time available for keeping in some of your regular sessions.
Sophie Kirk - Triathlon Coaching 