I work on a 'delayed' drift (think the aussies call it a 'hustle, tackle, jam'). 

 Say we're defending against a set piece i.e. scrum, rather than getting the open side flanker to move up directly on their 10, we get him to move up on his inside channel to cover the BSW or 15 taking ball into that channel from an iside pass from the 10. our 10 moves up on their 10 until the pass is made out to the Inside Centre (IC), at this point, rather than our 10 drifting onto their IC he covers the channel inside of their IC where their 15 could break. Our IC moves up on his oppo, then drifts only when their IC passes outside and so on.

So, rather than calling a 'drift, slide, push' and everyone moving onto their outside man, we're always covering a possible break from the oppo 15., but as important is that if the centres keep the ball and our line speed is quick, we can effect a 2 man tackle on the ball carrier and the potential for turnover increases. 

Hope this helps... Den