The idea is to speculate on the special players (IF, SIF, TIF, etc.) that are in the packs only in an ephemeral way. It is a medium-term investment.
Indeed, the idea will be to invest in the end of TOTW (Team of the Week) on cards that should increase when they become rare, ie when we switch to another TOTW.
You should know that the TOTWs change every Wednesday at 19:00. If, for example, you invest on a TOTW 2 player. You buy it in the last days it's available in the packs (that's when it should be the cheapest) and you sell it for two Weeks later (at TOTW 4 in our example), when it becomes rarer on the transfer market and therefore more expensive.
Watch out for the players coming out of the TOTW. If you invest on an IF player and it will go out in SIF in the next few days, you must sell it quickly because its price will necessarily fall.