This stretch is great to do after doing some self massage on your hand when your hand feels tight or crampy. The deep inner hand muscles can get quite tight when doing high repetitions and especially with high loads with your hands and fingers. In the clinic we see many rock climbers that come in with various finger and hand injuries that also have elbow injuries related to this.
Start by making a closed crimp grip with the joints of your fingers, but avoid closing your hand so you don’t make a fist. Then place the meaty part above the heel of your opposite hand under the finger tips of your hand to be stretched, wrap your fingers over the knuckles of the hand and flex that wrist to extend what’s called the proximal Metacarpal joints.
This will give you a deep stretch into the hand. Hold this for 30 - 60 seconds doing 3 sets on each side.
If you’re unsure about the technique or have uncertainty about what you’re doing, consult your local Physiotherapist before continuing.