Balls of the Feet, Not the Toes, in the Split Jerk
A common mistake in the split jerk is to be up on the toes of the back foot. This is unstable in every possible way, and prevents you from supporting enough weight on the back foot to keep your balance approximately even between the two feet. Being up on the toes allows easy spinning of the foot and slipping.
The back heel should be raised off the floor, but the balls of the foot should be flat on the floor and in full contact - this is necessary to support adequate weight for a proper split receiving position and will greatly increase the stability of the back foot.
This proper position is easier to achieve if you keep the back foot relatively close to the platform as you split rather than lifting it excessively. This is aided by maintaining an upright trunk as you split rather than diving your head and chest forward under the bar. Focus on lifting and reaching the front foot and the back foot will largely take care of itself.