2 uncircumsized men stitch their foreskin together and make it form a cup. Then they wrap it around both their cocks, and cum inside of it until it fills up like a balloon. Then, using their dicks like a spoon, they slosh the cum around until it becomes foamy and then they drink it.
When you drink a milkshake that tastes heavenly enough to make you 'mmm' . Often accompanied by a smile, and some guy staring at you and thinking. 'What the f***.'
Some milkshakegasms aren't as drastic, whereas some can lead to jizzing your pants in satisfaction.