Be sure to get proper training before using this technique. Improper performance of the hook breath can cause the blackout you are trying to prevent. This is discussed in the introductory paragraph of the document below -
"The only drawback was the problem with some individuals holding the pressure (closed glottis) for an excessively long time. Holding a very-high intrathoracic pressure for 7 to 10 seconds, even with concurrent skeletal muscle tensing, can markedly reduce return of venous blood to the heart. This can result in loss of consciousness, similar to that which results when performing a sustained Valsalva maneuver (no concurrent skeletal muscle tensing). Complete closure of the glottis not to exceed 5 seconds was considered the upper limit of maintaining increased chest pressure." 
 James E. Whinnery, M.D., Ph.D. and Duane C. Murray, HMC, USN, Enhancing Tolerance to Acceleration (+Gz) Stress: The "Hook" Maneuver, Naval Air Development Center, 20 August 1990.