I noticed most of the chinese penhold players leave a gap on their forehand like on the pictore above. Can anyone tell me why they do this & what are the advantages&disadvantages of leavling some space between the handle and the rubber?
Every serious player wants to customise or tweak their own tools for the best performance. This is especially more important in penhold grip than shakehand grip.
In table tennis, technical control is as important as the ball's speed. Speed can be controlled by your swing speed, the rubber/sheet, body movement etc. On the other hand, technical control largely relies on the holding hand. For shakehand grip you mainly use your arm/wrist to control the blade's angle/speed etc. For penhold grip, on top of arm and wrist you also use fingers to "fine-tune" the techniques. This means the stability and touch of the fingers are critical.
Needless to say it is more stable and precise to control the angle of a hard wood than a soft rubber. That's one reason why usually penhold players leave a gap between the rubber and the grip (although occasionally some penhold players don't leave any gap but that's minor).
Japanese penhold grips by default have a specific gap between the grip and the space where the rubber is to be put on. Chinese penhold grips, on the other hand, don't have such gap before you put a rubber on. How wide the player wants the gap be is really a matter of personal preference. This can be troublesome when J-penhold players switch to C-penhold grip racket (like myself) because the different shape and thickness of the two types of penhold grip would result in different angle and even feeling, and therefore takes some time to figure out the most suitable gap size for themselves.
Another tweak commonly done by penhold players (I believe shakehand grip players too) is to sand/trim their rackets on the corners where their fingers touch the blade. Any new blade comes with hard, sharp corner. This hurts or discomfort the fingers, therefore it is necessary to scrape and than fine-sand the sharp edge. Again, how much they want to sand the edge is a matter of personal preference.