I don't mind when a horse pokes at my pockets in a gentle, sweet, hopeful way. But if they are coming in at my belly like a land shark thats no fun.
Same with head rubs - if a horse slowly brings their head in, telegraphs their movements, gives me the opportunity to brace myself and lean into it etc etc I'll let them rub on me. But if they swing their head around like a wrecking ball and try to cheese grater their anxiety into my chest that aint happening.
My 'bubble' is made of water and cornstarch - malleable to slow, gentle approaches, but a firm hard barrier to fast aggressive movements.