Currently, it is impossible to tell whether a spindle is idle or transmitting power. There should be a visual and possibly auditory cue.

• To render a block as rotating, a block entity is required. I would prefer to cut down on the number of block entities for performance reasons. Is this unfounded? Additionally, rotation would have to be synchronized between spindles.
• Sparkles (probably black or wood-coloured) are a good alternative that theoretically should not require block entities.
• Auditory cues could get loud and obnoxious.

Judging from the above, particles seem to be the best way forward.

The black smoke is good enough for now.