This shape can be turned into a thousand year ko. White has two occasions to turn it into seki. The more outside liberties, the less likely the ko will be played (and thus the more likely White makes it seki in the end).
The vital point of this shape obviously is . After
, any move by Black other than a results in death in gote. It is no use for Black to throw in at a either: White will set up the thousand year ko with b, sooner or later.
White can also settle for seki.
This way the position becomes a direct ko, which is an inferior result for Black. White can finish the ko by filling at - it would be a mistake to play one of the other inside liberties instead.
(Sorry if you think the name is really silly).
(My go club calls this "Truck shape" -JaredBeck)