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Paraphrasing: Egg whites naturally have a pH of about 8.7 to 9.0. Acid substances reduces the pH 8 thus decreasing the tendency of the egg proteins to coagulate with the application of heat. weak acids, such as cream of tartar, citric acid [lemon juice] and acetic acid [vinegar] stabilize egg-white foams by lowering pH which increases the stability of the foam. Lowering egg white pH to an optimal 6.5 is acheived with cream of tartar. This keeps the foam loose and elastic, and stable enough to hold air: this in turn allows the cells to expand well when heated. From an interesting book: Experimental Cookery From The Chemical And Physical Standpoint by Belle Lowe [1937]

My copy of RLB's Chocolate Fondant [RLB] 1 tb gelatine ½ c water ⅔ c corn syrup 1 tb glycerine ¼ c shortening 1 ts vanilla 6 ¼ c icing sugar 2 c cocoa. Sponge gelatine over water. Stir until disolved. Blend in syrup and glycerine. Add shortening to melt. Remove and add vanilla. Mix sugar & cocoa. Add gelatine mixture. Knead to a ball. Turn onto lightly oiled top and knead until smooth. Wrap and leave overnight Notes: Chocolate Rolled Fondant is easier to work withwhen made one day ahead