In many ways, ABDLs have bigger problems than babies. One of these bigger problems that affects most ABDLs is something that they wouldn't 'fix' lightly - the penis. If the penis is leaning over to the right or left, urine will flow towards the unpadded sides of disposables. If the penis is pointing into the diaper, there will be a backpressure during urination. Depending on the ABDL and the fit of the diaper, this backpressure might be painful.
More creative males might consider sliding the penis inwards to try to approximate not having one. However, short of a few steps that would be difficult to explain in the emergency room, it won't stay there. This leaves two positions; 'up' towards the belly button, or inverted 'down' towards the anus. Both of these options have their advantages.
Generally, when standing or sitting, 'up' is less likely to leak. This is because both gravity and the waistband will prevent leaks out the top. Alternatively, when prone, 'down' is less likely to leak. This directs urine towards the leg gathers which usually have better curvature than the waistband. This is also true when crawling, etc.
However, there are exceptions. For example, the penis might shift around if placed in an unfamiliar position. Additionally, wetting might be easier in one position than another. Easier wetting might give rise to more frequent wetting, less urine each time, and fewer leaks.
With multiple wettings, wicking will also a factor. Some diapers will benefit from the 'down' position, which could wick equally up towards the front and back. 'Flipping' from one orientation to another might help if wicking is poor. In practice, any diaper will leak before it's total capacity is reached.
'Up' will tend to bulge in front, while 'down' will tend to bulge between the legs or in back. A bulge in front might be visible through clothing. However, it might be dismissed as being well-endowed, erect, or wearing something intended to give that appearance. 'Down' trades the bulge in front for one between the legs. This telltale bulge will get even worse if the diaper slips down a little. A dangling crotch is less visible than a bulge up front, but is less ambiguous.
The location of the penis will also affect the location of the stuffers or doublers. Generally, things go further forward for 'up' and further back for 'down.' For 'up', two small doublers can be placed in a V-configuration to prevent leaks out the sides. When laying down, a doubler around the waist, just inside the waistband might help prevent leaks out of the waist up front.
For ABDLs, there is more to the experience of wearing a diaper than merely not leaking. Depending on the ABDL, this might involve wetting, being wet, messing, and masturbation. When wetting while seated or standing, the 'up' position might result in urine dribbling over the penis and scrotum. In the 'down' position, gravity would only cause this flow when laying on the stomach. When laying on the back, the trickling would be against the buttocks.
'Down' brings the penis closer to being in contact with feces. This might be an interesting sensation, but has biological risks.
Some ABDLs also enjoy the thought of being wet. 'Up' places the wet spots up front, where the wearer would be able to see them. With 'down,' the diaper might need to be changed while still dry up front. This might seem unsatisfying or even a little wasteful. As with the tricking, both orientations would offer the feeling of being wet, but it would be felt by different body parts.
The 'down' position might seem slightly neutering or emasculating. The penis is less accessible, and ejaculation is more difficult in this position. The inverted penis might not be able to become fully erect. A penis that is trying to become erect will impede urination and might pull the leg gathers open.
Both up and down have advantages and drawbacks. If you get used to using both, you'll be able to use the one that works best for the particular situation and diaper. Cloth diapers, with better side coverage and wicking, are less sensitive to penis position.
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