Here are the default options... 
If there is no excess video at the head or end of the clips to be transitioned, you will be prompted if Ask me has been set as the default.
Move clips to overlap can be a good choice but be aware that this may throw audio that's not linked to video out of sync.
If Generate freeze frames is chosen, then halfway through the transition the FROM clip will freeze and the TO clip will UNfreeze, with no sync loss.