This is the info you are looking for. Check the link to CNC tip: How does G61 work? to get your answer. It is not what you were thinking and is not needed for 99 percent of people.
Please correct me if I'm wrong anywhere here.......In all my years of CNC machining I have never seen nor had the need to use a G09 code. According to my information, (Haas programming manual) G09 is exact stop with deceleration. (I tried it but I didn't see any decel but maybe it's micro-seconds) It saids its a "one shot" G code so my guess it won't work in a canned cycle like a G81. Would a G09 be used in drilling & reaming dowel pin holes? (If anyone replies to this I'll lose a bet to my wife.)
Keeps from rounding corners during high feed rates. Hope you didn't bet anything you'll regret
G91 G01 Y100;
will not give you a sharp corner. The X-motor would start before the Y-motor stops completely. We define an in-position width through a parameter which determines when the other motor would start. G09 forces the first motor to stop completely, before the next motor starts. This, however, is normally not needed because rounding is very small. Knife edge may not be needed. Moreover, exact stop would increase cycle time.
Many small scale turned products call out very small fillet radii (.001" MAX.)
To do this, you need a sharp tool and an "exact stop" in the corner.
Think of it has the shortest possible effective dwell.
Sorry about your bet
Control the process, not the product!
Machining is more science than art, master the science and the artistry will be evident.