Fitted, the pressure of the blade on the glass tilts the hinge end of the 'cup' of the arm on the wheelbox spindle away from the glass:

With the arm off the glass the cup can be tilted the other way, and in this position the arm can be wiggled and pulled off the spindle:

This little clip inside the cup at the hinge end ...

... seems to lodge under the splined section of the wheelbox spindle to stop it flying off in use:

T Aczel in Australia posted these wiper arms - the outer two are TEX and the middle one an original. The TEX are completely different to the original, and are even different to each other. Having lost about four he complained to the manufacturer, their response? "If you don't like our arms we suggest you buy from our Chinese competitors"!