The basic mechanism of sexual sadomasochism is the following: response suppression, linked in some way to a sexual opportunity.
The response being suppressed can be an actual behavioural response, or it can be one of a set of emotional responses that includes anxiety, embarrassment and guilt.
The response can be suppressed internally, i.e. by deciding not to respond to something that one normally responds to, or externally, for example by means of physical restraint. The implication is that there exists some part of the brain that measures the non-occurrence of expected responses, and which is connected to those parts of the brain involved in sexual arousal.
The biological purpose of sadomasochism is to encourage the sadomasochist to do something that they would not normally do or put up with something that they would not normally put up with in order to make the most of a difficult or infrequent opportunity for sexual satisfaction.
"Shiny clothes" fetishism is a side-effect of sadomasochism. The responses that get suppressed are the movements that the eyes would normally make when looking at reflected images seen in the shiny fabric. These movements are suppressed in favour of those made in order to look at the fabric itself (which always has a residual matt component in its reflectivity), the shape of which reveals the sexually attractive shape of the body underneath the clothes.
A ticklish masochist, when tickled, normally reacts to the tickling by squirming, laughing and trying to avoid or prevent it. But suppose that his sexual partner starts tickling him. He may become sexually aroused as she tickles him. As he becomes aroused, his normally ticklishness seems to disappear, in that his normal responses to tickling cease to occur. But as she continues to tickle him, he becomes more aroused. If the situation terminates in orgasm, and she continues to tickle him, she can observe that his normal response to tickling, i.e. squirming and laughing, has very suddenly returned.