The Portable Kettlebell Sandbag™ represents a shifting-load that limits vertical manipulation of that load via the handle. I recommend Russian-style swings (chest to eye-level), or just above that. You should experience a fluid swing with the PKB bumping against the fist / grip before resuming it's downward arc.
You can still, however, train overhead swings with the PKB. Here is how:
1. Rather than gripping the PKB by the handle, grip it by the webbing straps below the handle.
2. Firmly rest your knuckles against the top of the bag as you tighten your grip around the webbing straps into a fist.
Please note: this type of grip may not prove as comfortable as utilizing the handle, depending on the weight of the bag and number of repetitions being performed. You may want to consider gloves, but the overhead swing is possible : )