While even a heavy, experienced masochist is almost certain to beg for relief well before damage is done through steady pressure alone, if you have any doubt whether you're injuring him, stop. Ball stretchers aren't hazardous within reasonable limits, but don't get impatient and overdo. Do not leave the any genital bindings on too long and certainly not overnight - a good rule of thumb is to remove them every 20 or 30 minutes and allow the circulation to return to normal for a while. See the Cock Torture briefing for more on the dangers of circulation blockage in cock and ball play. Don't combine this kind of bondage with any other strong stimulation that might cause him to yank on his balls involuntarily, in reaction to pain elsewhere, unless he's otherwise so tightly restrained he cannot move enough to put pressure on them.
Bruises generally heal by themselves, though an ice pack can limit swelling.
Medical intervention is not usually necessary unless the bruising doesn't fade normally or you suspect an infection. More serious is a hematoma , which occurs when an injury ruptures larger, deeper blood vessels and a pool or pocket of blood forms between layers of tissue, such as between the scrotum and the balls. The pocket of blood will generally clot in a short time and form a hard mass. Externally, it will appear as a firm, bulging, or swollen area. A small hematoma will usually be reabsorbed without lasting damage.