It always senses your local wind. Or strictly, the component parallel with the pitot head.
But, it has a bottom stop.
If the bottom stop is at 20, and you are pointed into a 15kn headwind, it'll lift off the bottom stop at 5.00000001kn groundspeed.
Roughly.
The other complication is that pitot-static systems are inaccurate below 1.3Vs and hellishly inaccurate below Vs.
So, whilst if it was perfect, in a 20kn headwind, indicating 10kn on the dial, you are reversing at 10kn - which is difficult in most light aeroplanes. This just illustrates that you can't trust anything an ASI tells you below Vs, and need to be very cautious below 1.3Vs.
G