Instrument proficiency checks (excerpt from FAR §61.57):

No person may act as pilot in command under IFR or in weather conditions less than the minimums prescribed for VFR, unless within the preceding 6 calendar months that person has for the purpose of obtaining instrument experiance in an aircraft, performend and logged under actual or simulated instrument conditions, either in flight in the appropriate category of aircraft for the instrument privileges sought or in a flight simulator or flight training device that is representative of the aircraft category for the instrument privileges sought-

(i) At least six instrument approaches;
(ii) Holding procedures;
(iii) Intercepting and tracking courses though the use of navigation systems.

A person who does not meet the above experiance requirements within the prescribed time, or within 6 calendar months thereafter, may not serve as pilot in command under IFR until that person passes an instrument proficiency check consisting of of a representative number of tasks required by the instrument rating practical test. The instrument proficiency check must be given by an examiner, a company check pilot, an authorized instructor or a person approved by the Administrator to conduct instrument practical tests.