This is a type of ergonomic grip where the thumb of your shooting hand points straight up instead of being wrapped round the stock of your rifle, whether it’s a standard sporter or has a pistol grip or thumbhole. Some shooters find it more comfortable as the thumb is lying naturally, with no strain on any muscles, rather than being forced to stretch round a grip where the thumb muscles are being used and are therefore at risk of fatigue.
You can do a simple test to see if it’s worth trying. Hold your shooting hand out in front of you and relax all the finger muscles. See where your thumb is naturally lying. For many people, with their arm outstretched, their thumb will be pointing in the same direction as their arm, rather than sticking out to the side, meaning the thumb-up grip could be more comfortable for them and offer more control.
However, even if you like the idea of shooting thumb-up, your stock may dictate otherwise. Some stocks will have a deliberate thumb shelf or groove built into them, and some are neutral and the thumb can be rested there, but others offer no provision at all. However, if you are able to adopt a thumb-up position with your particular rifle, it’s worth at least giving this type of grip a try. I was an instant convert, and now feel a little disgruntled whenever I encounter a stock that won’t let me adopt my favourite grip.