Any extension whether it be front back or sideway must come from the supporting foot. Flex, or if you like the word bend,Bend that knee towards you partner when moving either back or forewards. Without the use of the knee of the supporting foot your dancing will be doomed.This always worked for me. Moving backwards as in the Foxtrot. I stretch behind to the tip of the toe, move the moving leg and try not to lower the supporting leg. You will lower of course, its impossible not to do so. My partner and I plus many others that I know carry out almost like a ritual with an extended hold, Foxtrot walks all the way down the floor, then backwards in turn. It is the only way.Don't forget the golden rule. You should lower the heel to the floor of the supporting foot as the moving foot passes the supporting foot, not before.And don't forget the flexing of the knees.