The debate about what is a forcing move has been going on for a long time
And is totally misplaced. That is the nub of the problem.
I've no idea who first used "forcing move" in English, but whoever it was he did a disservice to western players. I imagine it was borrowed from chess, but the meaning there has next to nothing to do with go. In chess you use forcing moves to restrict your opponent's replies, thus making deep tactical calculations easier. It's a way of pruning the tree, in other words.
In go, when a kikashi arises there will be nary a tree in view, nor even a shrub or a sapling. Rather than horticulture, the focus is on pennies and pounds. Taking care of one will ensure a supply of t'other.
A typical attribute of a go position is that there is usually more than one way to use a move. A good player will wait as long as possible before deciding which way to play. The better the player, generally speaking, the more possibilities he will see, and for a pro this often translates into waiting even longer than even a strong amateur expects, but at pro level there are considerations that may force an early decision. For example, the pro may sense that the game is on the cusp of a major fight from which it will be difficult to break off. He may run out of time to play the kikashi. Or the style of play of either player may be such as to tend to reduce the possible ways to use a move. So timing is always an issue.
But, no matter whether a player has played a kikashi early or late, what has he actually done? He has not reduced the opponent's options. It's not chess. What he has done is to "cash in". He has taken what Japanese refers to as the "uchidoku". He has taken the profit while it is available, and irrespective of how meagre it is, because he doesn't want to lose it by shilly-shallying.
However, what amateurs often overlook is that the profit can be measured in more ways than taking points. The reasons for a pro making a kikashi move are more typically to do with things like overconcentrating the opponent, settling eye shape, removing aji and other nebulous things (of course it can include taking points as well...).
Let us look in some detail at a Japanese definition to get a sense of what is in the (Japanese) pro's mind: 利かし: 現在先手で打てる手で、しかもなんらかのプラスにこそなれ、損のない手を、現に打っておくこと.
Doing a sort of
explication de texte, we see that it mentions twice the fact that it is a move played "now" (現). This nowness is just what we associate with cashing in. It is a move that "can be played now in sente" (現在先手で打てる手). That is important because when we cash in we want to get profit while we still can - while we still have sente.
The final verb form used (ておくこ) implies that we have done and dusted. This isn't about the future. We've played our move and the various possibilities are now finished with. Again, this is associated with cashing in. We accept that we've withdrawn our investment bonds in favour of real wonga.
Another aspect of cashing in is that we don't want to lose what we already have, and we have this in the definition. It's a "move with no loss" (損のない手). In fact it's a little more than that. When we cash in we are nearly always doing that to secure a profit, however small, and this definition says the same thing: "(a move with no loss) -- indeed one with something extra" (なんらかのプラスにこそなれ).
Everything we need to know about a kikashi is there really. In practice there is a difficulty in that in Japanese it forms part of a nexus of related words (kikashi, kikaseru, kiki, uchidoku, kimeru...) and so we see various translations or even circumlocutions which contribute to a fuzzy picture. But the real problem is simply in the term "forcing move", with the connotations - appealing for amateurs but regrettably addling - of power and sente (and that too not in its proper sense - to too many people it just means getting a kick from "I made him answer").
My suggestion is that if you wish to study kikashi (and it's a hugely fruitful way to improve) you should set before you something solid and easily grasped, such as a tablespoon, a supply of poker chips, and a bottle of whisky or whatever you tipple.
Whenever you play over a game and come to a kikashi, if you think of the word "forcing", ding yourself on the head with the spoon. If you think of the word "sente", ding three times. (You won't lose the sente - it's implicit in the kikashi, but to learn you have to forget.) You could also try pricking your thigh with an awl, as in the traditional Oriental fashion and as used by the great Fujisawa Hideyuki, for example, but elf'n'safety considerations preclude recommending that.
But whenever you see the 'effect' (the kik- bit of the word) a
pro kikashi has, cash in a chip for that side. As the game proceeds, you may see the supply of chips on one side grow larger than the other side's. When that happens, you will have a clear representation of the alarm that a pro feels whenever he has been kikashi-ed. And of course you will start to understand why comments about ostensibly subtle kikashi moves tend to loom large in high-level commentaries.
And every time you see a kikashi made
and also realise the various options that have now disappeared, take a congratulatory slurp of your tipple.
If you eventually slide under the table, at least you will know that you now understand go much better.
is not as good because it metaphorically bumps your (stone's) head against the wall (costing you liberties) and indirectly leading to a situation where you allowed your opponent to Hane at the top of two stones, which is bad shape.