Select Step under Tools to display the Step dialog box.

Step This setting specifies the step by which the arrow buttons move the selected control points in the corresponding direction: hence, each time you click one of the arrow buttons, the control point moves in the direction of the arrow by a distance equal to the value of the step. Note that the last value you entered will be retained for the whole working session, even after the command is ended.
Click one of the arrow buttons: the control point moves in the direction of the arrow (+x, -x, +y, -y, +z, -z) by a distance equal to Step: each time you click.

Note
The Step option — as well as the Scale and Transform ones — are not available unless there are valid control points selected. For this reason, when the selection is reset the said options are closed.
Please also note that the mentioned options are mutually exclusive: any time you select a new option, the current modification will be completed and the new one will begin.