The photo above shows me crocheting when holding my crochet hook using the knife grip. Please be aware that this is not the way I typically work.
As you can see, I am a right-handed crocheter. When holding my work using the knife grip, I typically hold my crochet project in my left hand, and I also regulate the yarn and the yarn tension with my left hand.
Left-handed crocheters can reverse this. Click here to see a photo and more information about the knife grip for lefties.
For me, this works OK for small crochet projects, but not so well for larger crochet projects, where I prefer to hold the work-in-progress using both hands. When using the knife grip for traditional crochet projects, I haven't yet found a way of comfortably holding both my crochet hook and my work in my right hand -- because the crochet hook gets in my way.
Again, please keep in mind that this is only ONE possible way of working -- and there are many possibilities! It works for many crocheters, and it might work for you too. If not, no worries -- there is a way that will work for you! It's just a matter of you trying different things to see what's most comfortable for you.