To add onto what jingles said
I've found personally that backkicking and certain types of kicks don't work well with a relaxed foot, because it just bounces off.
If you want a relaxed foot to ghost hard, you definitely need a bunch of speed to make it ghost, otherwise the foot will just bounce off.
Most front on kicks, such as the most commen
1)Grab uke
2)Pull downward
3)Kick
with a relaxed foot, that works miracles, but if you're doing a backkick, if you're moving slower then probably 35ish~ you're foot will most likely bounce off, or get 1 dm possibly 2.
I've personally found in this situation that contracting or extending the foot before impact/way before the impact, you can get more then with a relaxed foot.
It's just messing with holding, extending, contracting and relaxing.
I've turned a 4dm boomhit into a 6 by just holding my foot a couple frame before impact with it being relaxed before ;o