Don't worry too much about it. I went into one of these a few years ago with no research done and no idea of what it entailed, and got a job out of it. It seems they are quite common for the large engineering and oil industries in the UK. You are right to look into it though - I think with a bit of planning you can show them what they want to see.
The technical assessment can involve testing on reading graphs, interpreting drawings etc. I have heard of hands-on tasks too, where you are given a mechanical gadget, given 10-15 mins to take it apart and examine it, then you need to explain to the tester what is the function of each part, and re-assemble it. For engineering companies, this section is quite important.
Group exercises are important - they want to see how you work in a team, do you lead or follow. They will probably have someone observe the team as you are working together to sense the dynamics inside the team. Natural leaders will stand out. Engineering companies sometimes give hands-on group exercises, e.g. the team is given an egg, paper, a balloon, string, various bits and pieces. From these things they need to design & construct some type of carrier to prevent the egg from breaking when it is released from the first storey window. All teams make a design and test them simultaneously....apparently the results are taken quite seriously....
Exercises can involve two teams negotiating a business scenario, each with defined objectives of what they want to achieve. They want to see how good a negotiator you are / how are your powers of persuasion.
IMO they are looking for:
- strong technical ability
- confidence in speaking and defending your position
- ability to work well in team, not necessarily to lead it, but that is plus
Push to get involved in everything / every discussion in the team. Don't be seen as passive.
They will often bring you all out for a night out afterwards - be aware, this is part of the assessment. Here you need to tread the middle ground, don't stay on the water to create a good impression, and don't tear the place up either. You need to show that you are easy-going, can have a drink and socialize with new people easily, but that the student binge-drinking habit has worn off.
How to prepare? I would think that there is little you can actually read up on or study....the technical part is either there already or it isn't. However, be in good shape, get there the day before if possible, have a good night's sleep before it. Concentrate on your attitude, from the start show a positive front, get involved at all times, give your opinion and defend it. Know the answers to the standard questions such as "Where do you see yourself in 5 yrs ?" etc. as this will come up in the interviews.
Good luck. If you need any more info, PM me.