http://literature.rockwellautomation.com/idc/groups/literature/documents/um/1769-um006_-en-p.pdf
Z Input Functions
There are three Z input functions: store, gate, and Z preset.
Store
The Z-input can be used to capture the current count value even when the counter is counting at full 1 MHz speed.
Gate
The Z-inputs can be used to gate (hold) the counter at its current value regardless of incoming A or B inputs. A gating function is typically one that lets pulses reach the counter (gate open) or not (gate closed).
Z Preset
Preset can be programmed to occur based on the actions of the Z-input signal.
Not sure how you have the reset wired/configured or programmed, but you can use the Z-input to store the current value.