Please can someone tell me what determines the glowing heads that sit on spikes in the starting arena. Each set of 2/3 spikes are above each players challenge bowl (where you get your rewards). I look frequently, sometimes i'll have five glowing heads, then 2, then none then back to 3. Pic below, the purple heads on the right for example there is two, on the left that player has zero.
What determines this? What is it? Please help.