last time i did this you just keep chugging, until the liquid gets to the narrow part, then back off a little and rotate the glass to let the liquid out slowly or it can end up your nose in your eyes and down your top.
the other one is a glass welly which i think is worse.
cos when i did the welly, we couldn't find a glass one so my mates used their dads old size 11's and they had not been cleaned