Stimming works by using something you control (motor neurons) to affect something you want to control (sensory inputs which feed into your senses and state of mind).
The reason this is desirable is that those neural networks can otherwise get caught up in activities you don't like. This difference, in turn, stems from the neurological differences underpinning autism itself.
To address the question... The underlying need might be met by either conscious or subconscious stimming (motor commands). I.e. you might get an intrusive thought and seek to regain control by doing a stim. Or, you might notice a high pitched noise has been annoying you for a while, then notice your hand (or whatever) has been doing some stim for a while, without you having thought about it consciously.
It follows, one often can change or suppress a stim consciously, however this has state-of-mind implications and the control itself is effortful and thus can contribute to depletion of the day's available cognitive energy. (Now I'm talking about masking / autistic burnout / spoon theory)
Experience of folks with severe traits may differ from what I've accounted here.