I'll try it graphically ....
.......--t1--------------------------------t2---------------------t3-------------------------> time
A ......K1-------------------------------K2
B ......K3------------------------------------------------------K4
You have two bones, A and B.
At time t1 you have a key for both of them (let's say this is frame 0)
At time t2 you move bone A and therfore produce keyframe K2
At time t3 you move bone B and therefore produce keyframe K4
Result:
- From t1 to t2, bone A moves from position K1 to K2 .
- From t1!!!! to t3 bone B moves from position K3 to K4 ==> B starts at t1, too!
Solution:
You need a keyframe (let's say K5) for bone B at time t2 wich is equal to K3 (e.g. by explicitly moving bone B to where it shall be at t2)
==> bone B does not move between t1 and t2, because K3=K5
.......--t1--------------------------------t2---------------------t3-------------------------> time
A ......K1-------------------------------K2
B ......K3-------------------------------K5---------------------K4
(Attention: Because of the so called "overshoot" bone B still could move a little bit, but the explanation for that is an issue for another lesson :-) )