Be careful not to straighten the knees by locking them back (you can press your hands against the back of each knee to provide some resistance); instead let them straighten as the two ends of each leg move farther apart. (yogajournal.com)
Then straighten your knee as much as possible and pull your leg toward your trunk for 5 breaths or so. (weckmethod.com)
The individual then straightened their knee over five seconds, held the knee straight for five seconds and then took five seconds to lower the leg. (stretching-exercises-guide.com)