Default [Q] no sideload parameter for adb

All the rooting guides for android use 'adb sideload'. I have the latest sdk, but there is no 'sideload' parameter.
It's not listed when I write 'adb --help', and it's not a hidden parameter either.

I've been searching on google, but it seems like nobody else have a problem, which doesn't make sense at all.

Thanks.