  Recently a hard X-ray transient event, Sw J1644+57, was discovered by the
Swift satellite. It likely marks the onset of a relativistic jet from a
supermassive black hole, possibly triggered by a tidal disruption event.
Another candidate in the same category, Sw J2058+05, was also reported. The low
event rate suggests that only a small fraction of TDEs launch relativistic
jets. A common speculation is that these rare events are related to rapidly
spinning black holes. We attribute jet launching to the Blandford-Znajek
mechanism, and use the available data to constrain the black hole spin
parameter for the two events. It is found that the two black holes indeed carry
a moderate to high spin, suggesting that black hole spin is likely the crucial
factor behind the Sw J1644+57 - like events.
