Spanking focuses on the buttocks and on submission. A spankee who arches their back and offers their bottom a whipping with the strap is submitting to their disciplinarian both psychologically and physically. Being anally penetrated by a dildo or a cock is, for many people, one of the ultimate acts of submission.
When a hard or punishment spanking is chosen in the Choose a Spanking application, there is an option to include a possible punishment fucking at the end of the spanking. For a boy spankee this punishment fucking will, of course, be in the ass. A girl spankee may receive her punishment fucking in either her pussy or her ass.
The "punishment" in an ass fucking should involve submission, not pain. Or at least nothing more than brief discomfort during initial penetration.
There are many posts on the internet about how to engage in safe and enjoyable anal sex. If you have not engaged in anal sex before, you should read some of these articles.
Most importantly, when performing anal penetration, use lots of lubrication. There are a variety of lubes available, some of which are formulated for anal sex. These lubes are thicker and tend to last longer.
Spankings are supposed to hurt, but anal sex is not. Forget anything that you have ever seen in a porno film. This is not reality.
When giving an ass fucking, take it slow. If the person you are fucking has not been fucked in the ass before, try penetrating them first with a lubed finger (latex gloves are good here). If there is pain when you penetrate them with a finger, then you are both going to have to work up to penetration with a cock or dildo. Using a condom your cock or the dildo helps keep things safe and can reduce friction.
Anal sex can be messy. If this bothers you, a water enema can be used to clean out the rectum before the anal sex session. This can be included in the spanking play. The disciplinarian can tell the spankee to get ready and then come tell them when they are ready to take their spanking.
If you are receiving an ass fucking, the initial penetration will be less uncomfortable if you push out. If you have cleaned yourself out before taking your fucking, you will not have to worry about this leading to something unpleasant. By pushing out you will open your sphincter, making penetration easier.
Communication is critical. Listen to the person you are fucking and pay attention to any discomfort they are feeling. The person getting fucked should tell their top about any discomfort the are feeling. Suffering in silence will not result in a good experience.
If you are a girl using a dildo in a harness to give an ass fucking, give the fucking slowly once you have penetrated your spankee's ass. Fast strokes can lead to friction that may be painful. If your spankee has not been fucked in the ass before, it might be best to fuck them first with the dildo held in your hand, before you use the harness. This will allow both you and your spanking bottom to get used to the dildo.