Absorbed ashes preserve eternal merit

במצות הרמת הדשן שהי' נבלע אצל המזבח להראות שזכות הקרבנות נשאר קיים לדורות.

Regarding the mitzvah of terumas hadeshen (the removal of the ashes), [the ashes] would be absorbed beside the mizbe'ach, to show that the merit of the korbanos remains enduring for all generations.

A portion of the altar ashes was set down and swallowed up into the ground beside the mizbe'ach rather than carried away, signifying that the zechus of every korban is never lost but stays permanently rooted in that holy place for all time.

וזה שכתוב יזכור כל מנחותיך ועולתך ידשנה סלה.

And this is the meaning of the pasuk: "May He remember all your offerings and accept (lit. find fat/ashen) your olah, Selah" (Tehillim 20:4).

The Sefas Emes reads "yedashneh" — connected to "deshen," ashes — as alluding to this very deshen of the olah that was absorbed by the mizbe'ach.

שדשן העולה שנבלע.

That is, the ashes of the olah that were absorbed —

The "deshen" of the burnt-offering that sank into the ground beside the altar is what the pasuk hints at.

נשאר הזכות קיים לדורות:

the merit remains enduring for all generations.

Through that absorbed deshen, the zechus of the korban is preserved permanently, carrying its merit forward to every later generation.

Summary: When part of the altar ashes were absorbed into the ground beside the mizbe'ach in the mitzvah of terumas hadeshen, it showed that the merit of a korban is never lost but endures forever. The Sefas Emes finds this in the pasuk "yizkor kol minchosecha ve'olascha yedashneh selah" — the absorbed deshen of the olah keeps the korban's zechus alive for all generations.