An
inversion element on some steel
roller coasters by
Bolliger & Mabillard. Typically seen on their
inverted coasters, a heartline flip rotates the
train in a very small-diameter
corkscrew, producing a rotation around the
rider's "heartline" in a style very much like a
barrel roll. The heartline flip also shows up as a possible element in
Roller Coaster Tycoon.
Source: http://www.faqs.org/faqs/roller-coaster-faq/part2/