Rechaka means exhalation in pranayama. There is quite an art to exhalation.
When exhaling listen to the sound. Like finely tuning a musical instrument the sound is carefully adjusted so that its vibration is perfect throughout exhalation.
The sound effects the energy body and indicates the amount of pressure present. Too much pressure on the exhalation causes an increase in upward force around the temples.
If the heart feels strain at any time during exhalation relax and break the breathing technique. Take easy breaths until the heart feels relaxed and rejuvenated. Then it is safe to begin breathing exercises once again but allow each breath cycle to be less strenuous and breathe for less duration each breath so that it is easier on the nervous system and heart.