The unit "Hertz" means "per second".
Anything happening with a frequency of 0.4 Hz is happening 0.4 per second.
Technically, you must push the swing 0.4 times per second.
But since it's very hard to push 0.4 of one time, you have to be
able to figure out that you should push once every 2.5 seconds.
(1 push / 2.5 sec) = (1/2.5) push/sec = 0.4 per sec = 0.4 Hz .