from the Congressional Research Service's 'Congressional Oversight Manual' :
http://www.fas.org/sgp/crs/misc/RL30240.pdfUnder the inherent contempt power, the individual is brought before the House
or Senate by the Sergeant-at-Arms, tried at the bar of the body, and can be
imprisoned. The purpose of the imprisonment or other sanction may be either
punitive or coercive. Thus, the witness can be imprisoned for a specified period of time as punishment, or for an indefinite period
(but not, at least in the case of the House, beyond the adjournment of a session of the Congress) until he agrees to comply. The inherent contempt power has been recognized by the Supreme Court as inextricably related to Congress’s constitutionally-based power to investigate.
Between 1795 and 1934 the House and Senate utilized the inherent contempt power over 85 times, in most instances to obtain (successfully) testimony and/or documents. The inherent contempt power has not been exercised by either House in over 70 years. This appears to be because it has been considered too cumbersome and time-consuming to hold contempt trials at the bar of the offended chamber. Moreover, some have argued that the procedure is ineffective because punishment can not extend beyond Congress’s adjournment date.