Something that looks like a plot device that the author came up with on the spot after writing himself into a corner, often being inconsistent with previous scenarios and firmly established concepts in the series.
Please note that while all asspulls are bad writing, not all bad writing is in the form of an asspull.
O MY RUBBER NEN isn't really an asspull, because post-death nen was established long before the Hisoka vs Chrollo fight and how the revival worked is consistent with the functions of Hisoka's power. However, it is bad writing due to how it was presented. Far too spontaneous, cheesy, and casual, it was presented up front as an "explanation" for Hisoka's survival. I feel that this went against Hisoka's character as a magician with lots of tricks up his sleeve. It should have been a huge event that shocked both readers and the characters in the series, with everyone being dumbfounded over how Hisoka escaped certain death.