What is the advantage of crown down technique:
Zipping is less.
Working length is less likely to change.
Better penetration of irrigants.
All of the above.
Crown-down technique/step back = telescoping technique: 
Coronal 2/3rd preparation with 15, 20, 25 H file GG drill to flare. 
Apical segment preparation with 10, 15 no. file coronal segment. 
Advantages: ZIPPING is less, working length less likely to change, better penetration of irrigant.