Mistress uses the small dildo to enlarge your opening for further intrusions.
Here are your commands, depending upon the implement at hand:
Anal speculum: crank it open to the next point of discomfort, but do not back it off.
Anal probe: fuck yourself with it, but only up to the beginning of the next largest section.
Dildo: fuck yourself with it.
Butt plug: rotate it, pressing it against your prostate gland.
Finger: fuck yourself with it, using the entire length of your finger.
Do this for 2 minutes or until your asshole gets used to the intrusion and you feel you are ready for more.