We report new observational results and insights in the energy release during transient events on sub-flare level, which seem to be a common feature of active region loops. Our work is based on multi-temperature observations obtained high above the limb by SOHO/ SUMER. We conclude that the energy input into the loop system is initiated at one and only one foot point by an asymmetric impulsive mechanism. This trigger does not seem to be connected with any bulk flow and there is no indication that the plasma in the loop is replenished or replaced. These observational facts rule out some of the heating models under discussion. The electron density, N_e however, increases significantly during such events. If no new material is added to the local plasma, then the N_e increase can only be explained by a rapid volume decrease, i.e., by an in-situ pinch effect, compressing and heating the affected plasma.
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