A tomato question!

The recommendation is to pinch (removing the top main shoot)  by nipping the growth off between fingers or using a knife) out the growing tip after the tomato plant has set four to five trusses.  This is to encourage the fruit on the plant to grow and ripen before the end of the season.  In practise, some just leave alone (me   ) or wait until the plants reach the top of the greenhouse and then stop them by pinching out the tip.

By the way, this isn't done with bush type toms.

I've never understood the concept of pinching out at N trusses. Depends how early (or late!) you plant, how vigorous the variety is (my Gardeners Delight are several trusses ahead of my other Tomatoes) and probably a host of other factors.

I pinch out in the greenhouse one month before first anticipated frost, to give time for the Tomatoes to ripen.  Or when the plants hit the roof and I can't squeeze any more vertical growth into the greenhouse (e.g. by lowering them down the canes / strings a bit)