To perform clasp knife, assume your maximum stretch and tense against it for as long as you can, with the most force you can manage. At some point you will more or less collapse and your muscles will give up resisting the stretch. This is seriously hard work, and some would say risky. I guess if nothing else works, it might be worth a try.
I’ve never tried going to complete muscular failure, but I do find longer tensions work well, but they are really hard work!
For more information see:
- Relax Into Stretching by Pavel Tsatsouline