When I half halt, I generally don't pull back on the reins.
I keep my hands in one spot but don't pull back [to keep a consistent length of rein and neck.] I close my thighs, then add my calf to keep the horse moving forward. If they try to ignore the half halt and run through it, I ask harder. If they still arent listening I do a down transition to halt.
If they don't understand what it means when I close my thigh, I will use it harder and harder until they come to a halt, then I will immediately take my thigh off, but not give with the reins.