I've drank a Flaming Humbaba, crushed 300 beryl and observed using energy even though there were several seconds left on the buff. Someone in Global said that they could crushed only 270 without losing energy. When I started crushing 266 ore late and the buff again had only a few seconds left energy wa salso consumed. Seems like any crushing will consume energy that finishes in the last 5+ seconds of the buff.