A latched relay is just a way of having a relay keeping itself on. Nothing special, really. I drawn it using a double relay; you can also use a second relay to switch the lamp, which in turn is kept on by the latching one. A momentary push button can break this circuit and switch off the light after the PIR signal has gone low.
A PIR will normally keep the line high for a certain amount of time (a few seconds to about 10 minutes), then turn it low again. You can not interrupt this, you can however adjust it through a pot present on the unit.
Make sure your button out of view of the PIR sensor or just the motion of pushing it will trigger the sensor again.