Meh. Ok, so you keep sending a servo the same signal at 50 hz, right?
Now, let's say you are sending it 1000 us, so it's on one extreme. If you send start sending it 2000us instead, it will move at full speed at the other extreme.
You want to control this speed.
So, instead of sending it 2000us, you will send it 1005, and wait 5 times the frequency. Next, you send it 1010 and wait 5 times the frequency. You the send 1015 and wait 5 times the frequency. You will reach 2000 slower than maximum speed, thus, you have obtained your goal: controlling the servo's speed.
The increments between successive steps (1000, 1005, 1010 or 1000, 1001, 1002) and the delta t (time difference) between signal changes will determine the final speed.