The patented one way valves let the air in preventing vacuum creation but do not let any liquid out. (babygearhub.net)
[1][2] It is distinctly different from standard VVT (variable valve timing) systems which only change valve timing and do not change the camshaft profile or valve lift in any way (en.m.wikipedia.org)
Its shape is easy to hold, the flexible nipple and petal design make an easy transition, and the two - way valve system reduces air intake and colic. (mymilkybaby.com)