This video demonstrates the solo version of the biceps–forearm stretch. This is where your strength can really help: once in the floor biceps stretch, you clench your fist, flex the wrist as much as you can, and while maintaining the maximum flexion, you internally rotate the whole arm, from shoulder to wrist.
Take your time, and don't forget to completely relax the tummy (without letting anything move) before taking in another breath and re-stretching. Once in the end position, the key instruction here is to let the whole body go as soft as you can.
Find the audio file of this video at https://supplementary-material.s3.us-west-2.amazonaws.com/Kit+solo+floor+forearm+and+biceps+stretch.mp3.