Beyond stretching, there is a voluntary deep relaxing of muscle which can occur. For example, if the muscles around your hip are tight and you stretch them you may get a stretch-pain sensation, combined with end of range (physical blocking of movement) and discomfort becoming unbearable this is what limits you from moving deeper into the stretch.
If you have a basic connection between your mind and body you can use your attention to ‘ask’ the muscle to open and relax.
While in the stretch and feeling the muscle stretch sensation you ask the muscle to relax, the same way you would tense (contract) your muscles you do the opposite. The best way I can describe how to do it is to say you put your attention in the muscle and soften or relax it.
Visualisation can help here too if you are trying to visualise it as well as feel the muscle open up and the fibres unwinding and spreading apart. This will help to cause more effectiveness of your decreasing of muscle tension or gaining a greater range of movement.