You can see here for example https://youtu.be/mKn4Pi3qXjU?t=3594 that members of the Bundestag holding cards up during a vote on a bill. Is this a special kind of voting? How does it work?
The use of cards is a rule designated by Rule 52 of the Rules of procedure to document members who don't participate so that they can be fined for not participating. The normal voting procedure (that is the one NOT designated by Rule 52).
allows precise figures on the results of the vote and the majorities obtained to be recorded, but does not indicate which Members were absent. It is therefore common in parliamentary practice for a vote using voting cards bearing Members' names, in accordance with Rule 52 of the Rules of Procedure, to be demanded in such cases. Under this rule, instead of the vote being taken by a show of hands, all Members cast their votes using a voting card bearing their names and their vote ("Yes", "No", "I abstain"). The name of each Member together with his or her vote is then published in the stenographic record of the sitting. In addition, Members who fail to participate in a vote of this kind are penalised by having a certain amount deducted from their expense allowance.
I cannot identify the people actually holding the cards in the video (this is probably simply me being unaware of the faces of the important people). However, I recall the general idea from a visit to the Bundestag I participated in back around 2008-ish.
What you are seeing is a Namentliche Abstimmung: a vote in which each MdB's vote is recorded alongside their name. This is not the standard way of voting in parliament. Usually, a simple show of hands is used, the rule being that if everybody present at the time of voting who belongs to the same party votes the same way, the vote will be recorded as if the entire party had voted that way. Thus, usually you will see something like 'the groups of SPD, FDP, Left Party and Greens voted in favour of proposition 123/something; the CDU/CSU and AfD as well as independent members voted against'.
In some cases, parliament decides that a vote by name should be used. For those cases, MdBs have small cards with their name on like the ones you can see being held up in the video. These cards come in blue (in favour), red (against) and white (abstain). Voting is conducted by every MdB throwing one of their cards into a box; the cards are then counted, the names recorded and the results made available in the stenographic records.
It is common practice for MdBs not to be in the plenary hall all the time during business weeks. Instead, they will often work in their offices on the premises especially if the current debates do not touch their areas of expertise or are less relevant to their constituents. However, they have to vote in these recorded votes by rules of procedure, so they might hear the bell go off and be called in unprepared, not knowing which actual question is being voted on.
This is where the people holding up cards come in. They are representatives of the different parties and they hold up the card which corresponds to the parties' preferred outcome of this vote. In the example I gave above, you would see an SPD, FDP, Green and Left Party MdB standing around holding up a blue card while an MdB from the CDU/CSU and AfD would each hold up a red card. (Independents need to know their own stuff.) This advises members what the party position is, making the voting process easier for those less aware of what is being voted on right now.
The cards being held up are recommendations. Constitutionally, the MdBs are constrained only by their conscience and are theoretically able to vote as they please. In practice, as the outcome is recorded individually for each member, backbenchers who defy party lines too often or on too 'important' issues may risk not being considered for re-election.