A coolant interlock was a sealant on Federation shuttlecraft, used to keep plasma from leaking out of the nacelles. Opening the coolant interlock is dangerous and can cause the plasma to ignite leading to an explosion. Opening the coolant interlock is the final step of the Kolvoord Starburst which is illegal.
While undergoing an investigation of the death of Cadet Joshua Albert in a shuttle accident in 2368 on the USS Enterprise-D, Lieutenant Commanders Geordi La Forge and Data discovered that Wesley Crusher's shuttle's coolant interlock was left open while he was flying it, which they both found unusual. Captain Jean-Luc Picard realized that Crusher and his team, Nova Squadron, deliberately left their coolant interlocks open, so they could ignite the plasma and perform the "Kolvoord Starburst", a maneuver banned from Starfleet Academy since the 23rd century. Unfortunately, this attempt at the maneuver was what caused the death of Albert. (TNG: "The First Duty")