What is the advantage of crown down technique:
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.
Zipping is less.
Working length is less likely to change.
Better penetration of irrigants.
All of the above.