Gravity valve
valve, used by some players instead of the popping strap (Uilleann pipes)
Device at the bottom of the chanter, opening when the chanter is lifted from the knee; Used by some pipers instead of the Popping strap; Some players prefer it, others consider it to slow, and are therefore of the opinion that it inhibits their Popping style.
O’Canainn, Tomás: Traditional music in Ireland [1978]. London, 1979 (2), p.86.