The very term surrender implies a being. One cannot surrender to a Force, one can open to it and receive it. It is like the difference between prayer and aspiration. Prayer is always directed towards a Being or the Being whereas aspiration is impersonal. Similarly Force works in an impersonal way. When it enters a system then all that matters is whether the body and brain can hold it or not. If the capacity is not there one can break down under its pressure.
On the other hand surrender implies that the Divine decides on our behalf as to how much we can receive and how can we be made ready for more. It is a personal element that is introduced in the sadhana. Whereas when we directly draw the Force (we may use the word surrender but essentially it is a drawing upon the Force) then, if at all it comes, it will flow impersonally. If we have the necessary capacity we can sail through it and grow. But if we do not have then we can suffer a cracking down of the adhara.
That apart there is also the big challenge of being vigilant and awakened enough to be able to discern whether one is opening to the Supramental Force or to some vital forces since hidden behind there is an ambition to be a yogi or a supramental being. The Force will unearth these hidden tendencies and in the absence of surrender we will not know how to handle the onrush.
On the other hand surrender to the Divine Being (in our yoga to the Divine Mother in Her embodiment) makes the process safe and the journey smooth. Besides if we look closely at ourselves and try to discover as to what is the problem in surrender to the Divine Mother (after all if the goal is as this yoga promises the most logical thing to do is to follow the Guidance of the Master), then we shall discover that there is often hidden behind this pretext of surrendering to the Force the ego-self which is not yet ready to abdicate its small personality.
A lot more can be said but a few quote from letters of Sri Aurobindo on the subject make it verily clear about this issue.
LETTERS OF SRI AUROBINDO
About myself and the Mother,— there are people who say, “If the supramental is to come down, it can come down in everyone, why then in them first? Why should we not get it before they do? Why through them, not direct?” It sounds very rational, very logical, very arguable. The difficulty is that this reasoning ignores the conditions, foolishly assumes that one can get the supramental down into oneself without having the least knowledge of what the supramental is and so supposes an upside-down miracle — everybody who tries it is bound to land himself in a most horrible cropper — as all have done hitherto who tried it. It is like thinking one need not follow the Guide, but can reach up to the top of the mountain from the narrow path one is following on the edge of a precipice by simply leaping into the air. The result is inevitable.
The correspondent asked, “What makes the surrender to the Guru so grand and glorious as to be called the surrender beyond all surrenders?”
Because through it [surrender to the Guru] you surrender not only to the impersonal but to the personal, not only to the Divine in yourself but to the Divine outside you; you get a chance for the surpassing of ego not only by retreat into the Self where ego does not exist, but in the personal nature where it is the ruler. It is the sign of the will to complete surrender to the total…
No [surrender to the Divine and surrender to the Guru are not two different things]. In surrendering to the Guru, it is to the Divine in him that one surrenders—if it were only to a human entity it would be ineffective. But it is the consciousness of the Divine Presence that makes the Guru a real Guru, so that even if the disciple surrenders to him thinking of the human being to whom he surrenders, that Presence would still make it effective.
Yes [surrender to the formless Divine would leave parts of the being subject to the gunas and ego]—because only the static parts would be free in formlessness, the active nature would be still in the play of the gunas. Many think they are free from ego because they get the sense of the formless Existence, they do not see that the egoistic element remains in their action just as before.
It was not my intention to say that this way is the only way and sadhana cannot be done otherwise—there are so many others by which one can approach the Divine. But this is the only one I know by which the taking up of the sadhana by the Divine becomes a sensible fact before the preparation of the nature is done. In other methods the Divine action and help may be felt from time to time, but it remains mostly behind the veil till all is ready. In some sadhanas the Divine action is not recognised; all must be done by tapasya. In most there is a mixing of the two, the tapasya finally calling the direct help and intervention. The idea and experience of the Divine doing all belongs to the Yogas based on surrender.
The surrender must be to the Mother—not even to the Force, but to the Mother herself.