It’s currently unknown what Impending does. Maybe a new take on Suspend? But I can’t imagine what they’d change about it. I thought maybe it might be another Morph variant that enters with counters and unmorphs when the last one is removed, but I don’t think they’d have a 1GG cost with that
The conservative guess is it ETBs on cast and then after the “suspend” ends. The pushed guess is it enters every time a time counter is removed.