The spherical helmet that Barbarella wears with her environmental suit can change from completely reflective to completely translucent. To do so she reaches with both hands and touches controls (never shown) on the back of the helmet to activate the transition. Over 40 seconds the reflectivity withdraws into the base of the helmet, revealing Barbarella’s face through the glass.
Direct exposure to most of the electromagnetic spectrum is dangerous. To avoid Barbarella’s accidentally frying her own head in space, the suit must be designed against accidental activation. The strategy shown is called two-hand trip, which requires two hands to touch different controls at the same time to start the process. This is most often used in machines where you want hands out of the way of processes that could pinch or cut, but that aren’t dangerous after the process begins. In this case it’s less about mechanical danger than the risks with exposure.
Another strategy would be to use two-hand control, which would require constant contact during the transition. But since this transition is so slow (and presuming there is some undo mechanism that we never see) having this “two-hand trip” is not disastrous. If something or someone accidentally tripped it, she has more than enough time to recover.
On the other hand, 40 seconds is a long time for anyone to wait for things in the days of switchable glass. If your Barbarella was less dreamy-eyed & patient than this one, you might have to make a different tradeoff.