To raise less then the previous bet, which in only allowed if the player is going all in

When a player raises a smaller amount than the previous bet, which is only allowed if a player is going 'all-in'

This occurs when a player raises a prior bet but has to go all-in to do so. If the player under-raising - going all-in to raise - has less than - of the expected raise for that betting round, the betting round is locked. The term locked here means that any player who has already acted in the round (checked, called, or raised) may no longer raise. They may only call or fold. However, players who have yet to act (betting has not reached them yet) may raise the expected raise for that betting round, after calling. If the under-raise is half or more than the expected raise, the lock rule does not apply.