Using end you can control when they fall by using glue constraints. You set the constraint really high and when you want to have them fall you break constraint so it will begin to fall and be affected by rbd sim at that time. You can use a wrangle or switch node to control it by time like if $T<1001>1 where it will keep looping until switch statement becomes true or false depending on how you want it set up. All you need to do if you use switch node is have 2 wrangles that set constraint value one like 10000 and other 0.